If he doesn’t really think Luffy would take it alone, he would say they need to get up ASAP.
Aside from getting to the roof is easier said than done and would take up time, he can't fight, and the Scabbards are injured too so they can't do much vs Kaido.
It's not that he thinks Luffy can't win, just that he knows first hand how difficult the fight with Kaido is but has faith whatever happens Luffy will be able to come out on top, I think that's different to believing Luffy doesn't need any help, but right now he and the others can't help him.