Despite it not being a stun and merely being a slight displacement of a couple meters it can completely interrupt the cast of Apollo's ult and put it on cooldown. I may be wrong on this but I'm pretty sure there are several abilities in the game that do knockback and knockup effects that do not cancel Apollo's ult cast. Lash's Ground Strike, Dynamo's Kinetic Pulse, I'm fairly sure even Venator's Gutshot (so long as it doesn't get the wall stun, which would be a legitimate interrupt) can move you during it but doesn't interrupt Itani Lo Sahn's cast, but Silver's Boot Kick seems to be an outlier. It also seems a little unfair that her 2 ability in powered-down human form can interrupt an ult like that so I was wondering if this interaction is even intentional - thought I'd bring it to your attention.