Gpl licences don’t mean free as in no payments allowed, but free as in freedom. Many open source, even gpl, software is paid. All agpl, or gpl in general, means is that the code is free to copy as long as one abides by a set of pro-consumer rules while making your changes open too. It gives specific rights to the user or person who copies the project

This means you could build or host this product for free if you wanted, taking the code and setting it up yourself, but their distribution of the code or hosting of their project can include a paid tier or payment upfront. As long as they give all code for the project to people who use it they’re not in violation of the agpl