As far as I can tell the reason creators put extended videos on alternative sites (not sure why you're being so coy about what service you're talking about) isn't that they make more money there, but as an insurance policy since YouTube is a single point of failure for them. Making subscriber-only videos on YT doesn't help with that at all.
They do make that possible (Channel Memberships, https://support.google.com/youtube/answer/7544492).
As far as I can tell the reason creators put extended videos on alternative sites (not sure why you're being so coy about what service you're talking about) isn't that they make more money there, but as an insurance policy since YouTube is a single point of failure for them. Making subscriber-only videos on YT doesn't help with that at all.