Was Anti-Japaneseism a real political ideology?

Apparently 'yes', albeit a very tiny fringe position. The main thrust within this movement was indeed 'anti-' and directed within Japanese society on itself from a leftist perspective, but not as thoroughly 'self-exterminating' as in ' direct extended mass suicide' as the quote may seem. Although the original formulator of the very idea may have it intended as such.

A better comparison may be found in "Higashi Ajia Hannichi Busō Sensen [EAAJAF], which like Aum essentially set out to destroy the fabric of Japanese society." Including the traditions and values, relations within and outward that society…

But what is this ‘anti-Japan’? We are only too familiar with anti-Japanese (hannichi) sentiments in Korea and China, but this is nationalism expressed as antagonism towards a neighbour. The Left has long argued that Japan’s past militarism had tarnished its social structures and was a reason for breaking up the country’s residual elements of feudalism, monarchy and class.

In effect, Japan’s historical sins gave the communists and socialists the moral high ground. e ‘anti-Japan’ of the New Left went further and argued that the very existence of Japan itself should be questioned.

This ‘Anti-Japaneseism’ (hannichiibōkokuron), as it was known, advocated the destruction of Japanese society, wiping ‘Japan’ as we know it off the face of the earth.

Its proponents were not aggrieved Ainu or furious Okinawans, though, but Japanese, tying them in a rather curious double bind: what to do about themselves? is contradiction ultimately makes them a sort of Japanese version of the self-hating Jew.

[…] The pedigree of the Japanese may well be Korean or Chinese. It is highly disputable that they are ‘native’ to the islands they now possess. The exact nature and process of the immigration is unknown, though one archaeologist, Namio Egami, proposed a controversial theory in 1948 that unwittingly played into the arguments of Anti-Japaneseism, particularly after it was published as a book in 1967. Citing evidence of the sudden appearance of horses in the Kofun burial mounds period, Egami hypothesised that the Japanese are descended from a race of conquering Eurasians, continental equestrian invaders that galloped their way rst down the Korean Peninsula and then crossed over to Japan. From their very genesis, the Japanese have been invasive and bellicose marauders, constantly looking to a ack and vanquish neighbours.

The slightly 'less radical' version of that theme that saw some action argued

In the eyes of the Higashi Ajia Hannichi Busō Sensen, the post-war period was just as invidious and expansionist as Japan’s previous colonisation of Asia and Hokkaido, using investment, trade and overseas manufacturing bases to extend Japanese corporate interests. Harahara tokei asserts: ‘the “economic, technological and cultural” despatch, selling off under the label of overseas technological co-operation and so on, and the tourists “vacationing” to Korea to buy female entertainers [kisaeng], are all first-class aggressors of the Japanese Empire.’ In 1974, the Vietnam War was also still going on and Japan had been a tacit collaborator in the conflict, while Mitsubishi was manufacturing parts for the arms American soldiers were using.

The Marunouchi bombing was an extreme form of the wider hannichi ideology in the Left regarding Emperor Hirohito, Japan’s past crimes in Asia, the repression of ethnic minorities like the Ainu, contemporary corporate aggression, and more. Higashi Ajia Hannichi Busō Sensen was the most radical among its hannichi peers, as well as perhaps the most dangerous domestic New Left group at the time.

[…]

Over the two years there were thirty-seven incidents, killing fourteen and injuring over 400.14 Higashi Ajia Hannichi Busō Sensen, for all the idiosyncrasy of doctrine, was just one of many militant factions operating in the period.

The front carried out a further six bombings in 1975, but the more they acted, the more risks they took in being seen or leaving clues behind. Police monitoring an Ōta-connected publication began to tail suspects, which led them to the cells. They swooped in May 1975, arresting almost everyone at different locations at the same time—seven members, together with Mariko Arai, a suspected conspirator who was held for many years on the slightest of connections to the cells’ activities. Nodoka Saitō was arrested but committed suicide by poison soon afterwards while in custody. All of the members of the cells carried cyanide with them in order to kill themselves if they were caught, though only Saitō was able to take his capsule successfully.

Members like Masashi Daidōji who was the leader of the Higashi Ajia Hannichi Busō Sensen (East Asia Anti-Japan Armed Front) can be seen in connection Norio Sasaki, who was a member of this group and the Japanese Red Army.

The originator of this idea Katsuhisa Omori is listed as a 'copycat' perpetrator on the English WP article about the EAAJAF. In 1997 he himself described the idea a 'devilish', now embracing 'liberal thought' (conservative).

Quotes from:
— William Andrews: "Dissenting Japan. A History of Japanese Radicalism and Counterculture, from 1945 to Fukushima", C. Hurst: London, 2016.