Luffy was low diffed by Lucci in Water7. Few moments after he defeated Rob Lucci
He wasn't serious and he noted he was hungry then.
The World wasn't accepting Blackbeard as a yonko despite him conquering territories
The moment he overwhelmingly defeated Marco and co, he was accepted as a yonko
So what? Teach was already a powerhouse by the time he took WB's fruit, so it makes sense by the time the timeskip has been underway he'd be able to defeat Marco. Prior to becoming Yonko, the Gorosei were already saying that Marco and the other Yonko are probably the only forces who could contend with him, showing he was already considered in that level before the timeskip even started, he just hadn't gotten the political influence and infamy that earns the title of a Yonko, which he did during the timeskip. The Gorosei also noted he was already taking territories, so Teach definitely had a lot of WB's territories under his belt by the time he fought Marco. Teach was already in the same breath as Yonko before gaining the official title.
You mentioned Teach is a member of the Worst Generation but you failed to note that Teach is also within the age range of the other Yonko and was secretly building his strength because his plan was to become a top tier in secrecy. Teach has decades of piracy over the other members of the WG and is only included in that title because he broke out the same time as them, and like them the major events of the world in the year preceding the timeskip revolve around him in some way, not because he was within their age or strength range.
Teach doesn't have the same strength building rate as the Worst Generation. He has decades of piracy on a Yonko's crew over them, who are 100% rookies. Teach isn't playing catch up with them, Teach was in the general area they were trying to catch up in from the get-go, and rose into the tippy top in the timeskip.