Fastmail is a webapp, and it's very obvious. I use it out of laziness, because I don't use email on my phone that much. I wish they'd make a proper mobile client, but I don't think they care, since it's been like this forever.
But no, it's a formal android app and I offer two pieces of supporting evidence: the resource browser shows the apk very obviously has a bazillion android resource files <https://imgur.com/a/pUbQ8Bp> and it supports native fingerprint auth
Native fingerprint auth can be implemented on top of a webapp; that isn't evidence either way.
Either way, no offline access in an email app... pretty bad.