The message that they will support native ads was listed since Dec. 10 2018, the first version.
Also even though it is called firebase_admob
and it may seem like it is directly constructed for Flutter if you look at the GitHub repo readme, it shows that it is part FlutterFire.
[firebase-admob] tag exists while [admob-flutter] on Stack Overflow which makes it seem like it would be harder to get help when using since both of the packages don't have a proper API doc
It seems like firebase_admob
would be better than admob_flutter
as it seems to have a bit more features and can be more trusted to be maintained and up to date.
Update Edit: Now google_mobile_ads has been released with native ads for flutter.