the proxy is open source by design, the code is in the notebook I linked. part of webcode's aim is to run publicly auditable code, extending open source to the server in a new meaningful way. The webcode runtime is a bit of a binary blob, you can see its code
here and soon you can self-host it, but the point of fetchp is that is it is implemented in userspace service, using code hsoted by an independant 3rd party (observablehq) so you can kinda trust its not doing any shennanigans coz you can see the code, or fork the notebook for your own implementation of fetchp