I’ve been finding this to be the case with the fediverse so far. I have different accounts on Mastodon and Bookwyrm, since Bookwyrm is more tailored to book reviews and quotes. But I can follow Mastodon, Bookwyrm, Peertube, and Pixelfed accounts from one main “reader” feed, providing more of a mix than I ever got on one platform alone.
Agree, that's why I say in theory it's a good replacement. I don't yet buy it'll take off. I find it much more likely that we start seeing a bunch of walled gardens pop up instead. But if I'm wrong, then great.