Seems pretty reasonable to me. It's not 5 comments, it's 5 posts. I'm guessing they're trying to limit spammers and people with an agenda. As I recall, some Lemmy instances either implemented or considered limits on total posts across communities because of spamming.
Look ill be honest I don't really care.
Although 5 posts per day would indicate a need to limit rubbish, lighten the mods workload and perhaps limit AI post spam
Wish they'd do at least the first part over here. Most "power posters" never interact with the comments and are no better than an RSS reader. It's just posting for the sake of posting