They have a fee of 5%. Let's assume they have a price/earnings ratio of 20, that means they expect people to spend something like $450m/year on Patreon.
According to Wikipedia they are at around $150m/year already. I think it is totally feasible that they could increase that to $450m/year and not have to increase their 5% fee.
I'm a relatively heavy Patreon user; I donate ~$100/month to ~12 different people. The PayPal donate button doesn't solve the problems that Patreon solves for me. Those include: providing long-term support, managing my ongoing support in one place, finding out what the people I support are up to, and getting a notion of how much support they're receiving.
I still do an occasional one-off donation to somebody who doesn't use Patreon, but as far as recurring billing for long-term support, this is all I use now.
If you don't have your own website (like the many, many YouTube content creators), it's much easier to promote a Patreon link and get people to support you on a regular basis than it is to drive people to donate with PayPal. Patreon also offers novel ways to support creators, like tiered donations and per-creation donations. This level of nuance (and the management that comes with it) is hard to replicate on your own.
Are they structured as a payfac? If they are facilitating $100M in payments they probably pull in a couple million in revenue just from processing (assuming they charge for processing, I'm not sure)
According to Wikipedia they are at around $150m/year already. I think it is totally feasible that they could increase that to $450m/year and not have to increase their 5% fee.
Seems like a reasonable valuation to me.