score:8
This question is addressed in the Abhaya Sutta from the Buddha himself, on the topic of Right Speech.
Your question should fall under "In the case of words that the Tathagata knows to be factual, true, beneficial, but unendearing & disagreeable to others, he has a sense of the proper time for saying them." (Abhaya Sutta).
From your same link on the Mahatanhasankhaya Sutta, you can read the commentary by Thanissaro Bhikkhu:
The Buddha calls Sāti into his presence, and after ascertaining that Sāti will not abandon his view even when reprimanded by the Buddha himself, he abandons Sāti as too recalcitrant to teach, and turns to cross-question the monks as to the relevant right view of how consciousness functions in the process leading to repeated birth.
The Buddha's treatment of Sāti might seem harsh, but he is actually acting out of compassion for the monks in the assembly, in case any of them might be swayed by Sāti's position. Seeing Sāti as a lost cause, the Buddha doesn't want this lost cause to cause further losses among the other monks. We have to remember that during the Buddha's lifetime there were no written accounts of his teachings; the monks and nuns all had to rely on their memory of what they had heard directly from him or through word-of-mouth from fellow members of the Saṅgha. Thus the Buddha saw the need to establish orthodoxy whenever a member of the Saṅgha was espousing false interpretations of his teaching.
The Buddha also discusses this in the Kesi Sutta which is summarized here by Thanissaro Bhikku:
Once, when a horse trainer came to see the Buddha, the Buddha asked him how he trained his horses. The trainer said that some horses responded to gentle training, others to harsh training, others required both harsh and gentle training, but if a horse didn't respond to either type of training, he'd kill the horse to maintain the reputation of his teachers' lineage. Then the trainer asked the Buddha how he trained his students, and the Buddha replied, "In the same way." Some students responded to gentle criticism, others to harsh criticism, others to a mixture of the two, but if a student didn't respond to either type of criticism, he'd kill the student. This shocked the horse trainer, but then the Buddha explained what he meant by "killing": He wouldn't train the student any further, which essentially killed the student's opportunity to grow in the practice.
Upvote:4
Here is a case of the Buddha himself justifying his speaking harsh words with a for-their-own-good type explanation from the Abhaya Sutta:
Just yesterday, lord, I went to Nigantha Nataputta and... he said to me...'Come now, prince. Go to Gotama the contemplative and on arrival say this: "Lord, would the Tathagata say words that are unendearing & disagreeable to others?"... Just as if a two-horned chestnut were stuck in a man's throat: he would not be able to swallow it down or spit it up. In the same way, when Gotama the contemplative is asked this two-pronged question by you, he won't be able to swallow it down or spit it up.'
Now at that time a baby boy was lying face-up on the prince's lap. So the Blessed One said to the prince, "What do you think, prince: If this young boy, through your own negligence or that of the nurse, were to take a stick or a piece of gravel into its mouth, what would you do?"
"I would take it out, lord. If I couldn't get it out right away, then holding its head in my left hand and crooking a finger of my right, I would take it out, even if it meant drawing blood. Why is that? Because I have sympathy for the young boy."
"In the same way, prince:
(1) In the case of words that the Tathagata knows to be unfactual, untrue, unbeneficial (or: not connected with the goal), unendearing & disagreeable to others, he does not say them.
(2) In the case of words that the Tathagata knows to be factual, true, unbeneficial, unendearing & disagreeable to others, he does not say them.
(3) In the case of words that the Tathagata knows to be factual, true, beneficial, but unendearing & disagreeable to others, he has a sense of the proper time for saying them.
(4) In the case of words that the Tathagata knows to be unfactual, untrue, unbeneficial, but endearing & agreeable to others, he does not say them.
(5) In the case of words that the Tathagata knows to be factual, true, unbeneficial, but endearing & agreeable to others, he does not say them.
(6) In the case of words that the Tathagata knows to be factual, true, beneficial, and endearing & agreeable to others, he has a sense of the proper time for saying them. Why is that? Because the Tathagata has sympathy for living beings.1