They're primarily talking about Mastodon, a (originally a Gnu Social/Identica work-alike) platform that was started with blocking in mind. The creator, Eugen Rochko, is a fucking snowflake that can't handle anything that's outside his hugbox view of the world. Eugen has a blocklist that others subscribe to, and the mastodong dev community is known for being assholes. The other main fediverse player, Pleroma, doesn't really subscribe to that philosophy.