Well, MicroG is meant to be a drop-in replacement for closed-source Google libraries on your phone (i.e. Google Play Services / Google Services Framework) but obviously it will still have to connect to Google servers if needed (although you're able to control for what purposes). For instance, push notifications for apps often use Google's Firebase Cloud Messaging platform and MicroG allows the user to still receive those notifications in the absense of Google Play Services / Google Services Framework.

I suggest you have a closer look at the MicroG website where this is explained, too: https://microg.org/