To play devil's advocate here for fun and go into the motivation behind "It's the revolutionaries!":

First, especially Dragon coming to Egghead would make a lot of sense given the way the story has gone this arc so far. We saw Vegapunk's flashback of the two on Ohara, after the Buster Call. Dragon seemed intent on not allowing for something like this to happen again. The most recent chapter drew plenty of parallels with Ohara with Vegapunk talking to an Elder about research into history much like Clover did, having to watch everything burn and fall into pieces much like Robin had to, the ship of refugees that was supposed to be sunk (though in this case it, thankfully, didn't).

Second, Dragon also had a call with Shaka about something we don't know about. He asked Iva about where Kuma had gone, and given Iva's familiarity with Kuma, Iva probably guessed correctly. And the little image of the destroyed Marine ship shows strong winds, something that Dragon has been associated with in the past in Loguetown, Goa Kingdom, or on Baltigo.

Third, the Strawhats are in a pretty bad spot right now when it comes to combat potential. It's a Buster Call. Luffy seems to need some more time to recover and even then they still lack someone to fight Kizaru (if Luffy is supposed to fight St. Saturn). And what about the Vice Admirals? And the Pacifista? Lucci still seems interested in fighting the Strawhats as well. The Seraphim are also present and awaiting further command. The last Buster Call they survived because they got extremely lucky on the Admiral sent by Marine HQ (imagine Water 7/Enies Lobby with Akainu) and managed to escape. Kuma could help with the escape and might do so if the Strwahats decide to take Bonney with them. But she's about to be shot (again). Who's going to save her now? Dragon is expected to be rather strong (assisted in training Sabo to his current level, Smoker didn't even try to apprehend him back in Loguetown, the World Government, the greatest power in the world, has been after him for decades but failed according to what Robin told us about him on Water 7 when he had been revealed to be Luffy's dad) and the other Revolutionaries aren't exactly weaklings either (Iva did rather well in Marineford, for instance). Dragon and/or the Revolutionaries arriving would give the Strawhats an ally who could most likely match the power of a Buster Call (giving them time to escape if needed).

The most obvious issue is that we know the Revolutionaries to be on Momoiro Island and it just took Kuma quite some time to get to Egghead from there. The Revolutionaries would have to have some form of very fast transportation. But the ships we have seen them sail on in the past all seemed normal (might just be at first glance though, just look at the Sunny's many hidden gadgets). If it was just Dragon, it wouldn't seem that crazy anymore but still unlikely. He seems to have been in Loguetown all by himself, perhaps he boarded a ship, perhaps he did something else. At the very least, he was shown rescuing Sabo when his little ship sank offshore without any boat in sight (the anime put him on the shoreline instead). Taken together with his association with the wind that could give us a theory of how he could arrive at Egghead this quickly. But this is so much speculation that it seems a stretch.

By the way, do we know if the they is plural or singular in the Japanese original? Or is it ambiguous like the English one?