No, what was said was that “Groups are for Lemmy and hashtags are for Mastodon”.
That is to say that Groups are not for Mastodon, so Mastodon users should be content with hashtags.
But Mastodon users use Lemmy groups from Mastodon, and better group integration is already being planned by Mastodon themselves.
Ergo, groups are for Mastodon.
Having a Mastodon account means creating new habits. One of them is to check the originating server of an account. This is because that account may not be using Mastodon.