Wikipedia accepts all new entries by default. Almost all open source projects review any contribiution first before merge.
It’s also not fair comparison, because there can’t exists an encyclopedia you can learn from but not look what’s inside it. But you can obfuscate machine code, making it very hard to see what it does, so it’s more temping for code developers to put malicious features when noone can see it.
Lemmy is free software licenced under AGPL and that’s what matters.
I wouldn’t use proprietary app myself, but this is individual choice and those apps won’t hurt the ecosystem because they can’t break the server’s code.