Head of Let's Encrypt here.

We were aware of the "google.com.mg" cert soon after it was issued. We didn't revoke the cert for the same reason we don't revoke most certs: as far as we can tell, the cert was issued to the entity properly controlling "google.com.mg". Whether or not that is Google (the company) is not really within our purview.

That said, in this case, as a courtesy, we did notify Google employees and made the decision to report the site to Google Safe Browsing. GSB and SmartScreen are the right places to deal with things like this.

IIRC GSB did block the site for a while, but that block seems to be gone now.