Upvote:2
This website explicitly mentions strikes expected on the 15th and 16th. LyonMag quoted trade unionists saying the strikes at AviaPartner were going to "intensify". There is ample but patchy coverage of strikes by airport staff that were happening around the same time in the local press but very little about AviaPartner specifically.
At the same time, I wouldn't expect the local press to faithfuly report every day of industrial action in an ongoing conflict (the main local newspapers seemed to have barely covered AviaPartner, with only one article mentioning it in Le Progrès at the beginning of June). To me, reports of strikes before and after the 16th actually make the airline's account more credible. According to the reports I came across, only a part of the workforce was striking on any given day and not all flights were impacted so the fact that some flights did leave on the 16th does not seem very significant. The other cancellation you mention also lend the airline's account additional credibility.
Suing over this seems likely to be a waste of time and money.