They are the strongest of the current yonko yes. This chapter pretty much implied it by saying they are the most balanced of the yonko crew and singling out "Beckman", "Roo" and "Yasopp".
With that said, i won't say they are twice as strong. I expect Beckman's bounty to be 2Billions at best, could even be lower, since Blackbeard has a 2,247,600,000 Bounty while being a yonko with the influence of subordinates and allies as support to inflate it, and King's bounty might be in the 1.5Billions range, and thus not making Beckman's bounty that much higher. Yes i know bounty isn't power-level, but as a crew comparison, it helps in portrayal
Furthermore, as mentioned that's as far as current yonko goes, because when Whitebeard was alive, he was hyped beyond the current yonkos and since he was the closest to one piece, being able to be pirate king whenever he wanted, his subordinates could have filled the gap enough that they could be just as strong if not stronger than Shanks' subordinates
The fact that "Yasopp" who is one of Shanks' core member seem to have a beef with Vista, with Oden as a commander who could scar Kaido, Marco as the right hand man and Jozu in between further implies this. Ace also had pirate king potential. Teach ultimately became a yonko himself
So overall i think both Whitebeard pirates and Red Hair pirates are stronger than other yonko crews, but not twice as strong. They can probably high diff their respective counter-parts