This is the full panel:
They didn't fight because it's pointless; The more marines they fight, the more that come.
They aren't even obligated to fight the marines in the first place, so this makes sense honestly.
Blackbeard did the same thing with Akainu, but for another reason.
If he had fought Akainu there, they surely would have wrecked the battleship they brought with them, which would still leave them stranded without a ship, so again it's a pointless battle.
It's clear that they aren't AFRAID of fighting marines.
Hell, we see BB's crew fighting Garp despite him having a shit ton of hype.
They may have lost (besides Kuzan who was at a clear disadvantage but was still managing to get some good hits in on Garp nonetheless) for a while until Garp sacrificed, but you get the point.
We also see Roger not being scared of fighting Garp and Sengoku despite them being the strongest marines so far, Oldbeard not being scared of fighting the entire navy HQ, Warlords, Pacifista and Sentomaru to rescue Ace, etc.
I don't really care to argue about whether they're below or above these marines, we'll see very soon whether or not Luffy is above Kizaru and/or Saturn anyway (my money's on Luffy though for obvious reasons), but I just wanted to point this out because it's a particularly flawed argument.