Hello Alex,
My name is Jason, Jenny forwarded your question about Google Wallet Instant Buy to the Instant Buy support team.
You are correct, Google Wallet Instant Buy is not a payment processor. It is helpful to think of Instant Buy as a payment facilitator as it is designed for merchants who already have a payment processor/acquirer.
This is a quick overview of how accepting payments from Google users would work with Instant Buy:
Buyer selects "Buy with Google Wallet" on your website.
Your website would forward the purchase to Google.
Google would then charge the users account and place the charge amount onto a one time virtual credit card and send this credit card to you.
Then you would forward this virtual credit card onto your payment processor, such as Stripe, and the process would continue as if you had received a normal credit card.
More information about Google Wallet Instant Buy can be found here. I have also found this FAQ to be very helpful.
Please let me know if you have any additional questions about Google Wallet Instant Buy.
Best,
Jason