Like Google Fi, it’s a VoIP (voice over internet protocol) service so coverage should be the same as Fi.
Strictly speaking Google Fi is a MVNO. After losing Sprint/US Cellular access a while back nowadays they are a T-Mobile reseller e.g. you're getting access to T-Mobile's network and coverage with Google Fi.
For what it's worth my Google Voice phone number does show up as being classified as a VoIP number so that might be more along the lines of what you're thinking. The phone number I have on Google Fi does not show up as VoIP.