While it's likely true, GCM/FCM is handled by Google Play Services, which is whitelisted and has I believe system-level permissions. It has less restrictions than a regular app exempt from battery optimisations, as it's supposed to be the only app that needs to run in order to coordinate things like push messages or location updates across all apps on the device

edit: I missed that the original post was talking about keep-alives and you were probably referring to them. But how app-level optimisations can help with keep-alives?

TCP connection can be open and idle for hours without keep alives. This can easily be configured with kernel parameters:

https://www.tldp.org/HOWTO/TCP-Keepalive-HOWTO/usingkeepaliv...

I suspect the hard bit for normal apps is keeping the app active on the device while it is in the background. Google has their FCM receiver running as a low level service so can make sure it is always running.