Even if every single person used a FOSS client, someone would eventually come along and tell you why the one you use sucked....with a very long list of reasons why.
It's all new users. Maybe the dev and his fanboys activated their troll bots. Probably that's how they used to cripple criticism on Reddit. However they should be aware, that's not going to work on Lemmy.
It should also be noted that c/syncforlemmy announced the need for mods (after their vehement criticism regarding the pricing). Probably they want to play a dirty game.