It is anonymous but you're trusting the provider. The payment isn't tied to a page. No logs are kept. It's open source, but, in theory I could be running a version that does log or keep records but I have to reason to.
Don't forget that the payment provider (bank, cc, paypal) also has logs.. using the word anonymous here is a bit dishonest in my book.
I'm using it in the sense of
not named or identified
Not, if someone really really wanted to track down who wrote a page they couldn't.