Huh. I thought for sure Hydro Pump worked on DK's Up B and I swear I've seen it work before.
And why do people keep forgetting Shadow Sneak exists? Greninja is not easy to edgeguard, at all. He can very easily recover high with Hydro Pump and be far enough from DK so he cannot be punished, and Shadow Sneak is a teleport which we all know are great recoveries to have and it covers a great distance. The only way DK is going to edgeguard Greninja is if he recovers low but even then he can mix up his recovery to avoid a stage spike.
Try to edgeguard Greninja at your own peril.
Greninja's recovery is among the best and safest in the game so no, DK does not edgeguard him better than Greninja does the inverse. Greninja can keep DK away with b-airs and force an airdodge that could end up placing DK too low to recover, or he could get hit by the b-air and be sent to the blast zone depending on damage. Shadow Sneak off-stage is not to be underestimated either.
Greninja can combo DK really easily, can get out of disadvantage pretty well with Hydro Pump, is very hard to edgeguard and is faster with a good projectile. DK has survibility and power, but Greninja's shuriken force him to approach and play the frog's game, plus Greninja can punish DK's mistakes pretty well with his great dash grab. DK can turn the tables pretty quickly, I won't deny that, but I don't think he's going to have an easy time getting out of disadvantage against Greninja at all thanks to how mobile he is and how much he can combo him.
Greninja is not necessarily going to struggle to get kills either, Substitute kills ridiculously early and DK has enough laggy attacks for him to take advantage of. Sweetspot Up-Smash also has great KO power and b-air can very much spell doom for DK offstage. He can end DK's stock very quickly if the DK player isn't careful so his survivability is not as pronounced as it is with other speedster characters like Sheik and Pikachu who have more trouble killing.
I still think the MU is in Greninja's favor because he tends to have the upper hand most of the battle. I can't really say if DK's airgame is really better than Greninja's though I'm having my doubts about it, but overall I think Greninja is just really good at keeping DK at bay while forcing him to approach and play his game, what advantages DK has I don't think compensate for just how much Greninja has going for him.
That's what I think anyway, I'm hardly an expert on the subject of MUs but I don't think DK is going to have an easy time dealing with Greninja.