For communities yes due to cross instance stats but for instances themselves (which the stats above is based on) no. You can just use post read times in addition to the three which will catch anyone who has read a post. Post reads are something each instance has access to for its users so it can do the unread comments feature but it doesn't federate (but each instance self reports stats on itself).

It depends on what is being called activity

The standard (I say standard but its really just the thing most sites use since it boosts their numbers) that social media uses for monthly active users is to do people who have logged in. This is what mastodon uses as well

While they aren't actively contributing content they are still actively using the site (active account as opposed to dead account)

I think lemmy should match up to the mastodon and other social media calculations so these comparisons actually make sense otherwise were just making lemmy feel dead by calling a different calculation MAU than what people are used to and since both calculations are being compared like they're equal