EDIT: The script has been updated a few times. It now has new features. You should check it out again, even if you already have.
exactly what is says on the can. put in the multireddit URL of all your subs (explained in script), get a list of lemmy communities of the same name.
I find it good that you doubt and ask.
My answer (like OP’s one) is: You do not need to trust (blindly like for closed-source or platforms). The script is FOSS, you can check every step it does, before you use it. One of the biggest benefits of FOSS :)