I can't see Apple bothering to implement VP9 when
a) it is inferior to HEVC from a quality perspective
b) its only technical benefit, royalty free status, has been challenged by (admitted patent troll) Sisvel who have launched a patent pool with Dolby, Ericsson, JVC, NTT, Philips, Toshiba etc. patents behind them
c) its only practical benefit over HEVC is 4K Youtube...but they have already begun moving to AV1 for 1080p, 1440p, 2160p and 4320p.
I'd be pleasantly surprised to see it but I think Apple's general plan is AV1 > HEVC > AVC, and thats all there is to it. But they have been late to the open source party before (eg FLAC).
On the flip side, if Youtube "own" the app, they could very well have capitulated and done VP9 in software like in Infuse. But it seems odd for Apple to take credit for it at WWDC.
But speaking of which, also, AV1 software decode has been in Infuse for a good 9 months now.
Technical benefit aside, there are plenty of sites and plenty of content in VP9 and the *hardware* supports it, so they should support it.
Not supporting it just gives a far worse user experience, and apple are supposedly all about user experience...