I have never understood why people using MicroG can use apps like SNCF-Connect, while I cannot.

SNCF-connect is the largest online marchant in France. Since june, I cannot click the "pay" button anymore. It coincides with the new feature of Pay with Google.

So : is it because MicroG allows calls to Google services on e/OS devices, while my LineageOS refuses ? Or because MicroG implemented a try catch around this call, without any call to Google's servers ?

I think that's because /e/os also distributes App Lounge [1], which can connect to Google Store and handle payments (most of the time).

[1] https://doc.e.foundation/os/apps/app-lounge/

Some Play Services calls are stubbed out as no-ops and others are reimplemented either with fully free replacements (e.g. navigation) or else by calling proprietary Google remote endpoints (e.g. firebase)