Free Open-Source Software. Basically, an app for which the developers are like "here is the source code, you can check it, change it for yourself, feel free to play around"

The reason why this is important is security and freedom, as every developer can check if the app does anything spooky or just anti-user (like, send your info to Google and Amazon trackers, which Sync does).

Sync is not FOSS, that's a fact.

So yes, it is truth, not opinion. Sync is a double-edged sword: on one hand, it makes many more people come in, on the other, the general ethos of the Fediverse leans towards FOSS and Sync is not FOSS.