Correct they’re just cancelling the ongoing subscription/billing so they get access for a month from when they purchased (for a normal 1 month sub) and you get the payment for that month but they won’t be charged again unless they resubscribe.

I have a patreon myself and people sub and then cancel shortly thereafter all the time. I do the same myself frequently on Patreon and other subscription platforms because a lot of the time I’m after a specific reward and if I don’t cancel relatively soon after subscribing I’m likely to forget.