Alternatively they’d have to add VP8 support in their chips and one suspects they would be unwilling to spend silicon on that which could otherwise be used for whatever witchcraft their silicon designers are whipping up.
I’d grant that as a valid technical reason for limited video codec support. Silicon and battery are at a premium.
I'm sure a lot of them do, but it's also true that there are a lot of Mac laptops out there which will be upgraded to High Sierra that don't have hardware HEVC acceleration.
One obvious technical issue is that, as far as we know, there's no VP8 decoding hardware in any of Apple's products; implementing VP8 decoding in software might be more of a power drain than Apple wanted.
But what I found interesting in the WWDC session you linked to was that a lot of Apple products don't have hardware HEVC decode and\or hardware HEVC encode support. Apple has implemented software HEVC decoding and encoding in a lot of places. From that perspective, adding support for VP8 and VP9 wouldn't be much different.
