It's all timed, but there's no guaranteed date when Xbox and PC will get the content. It'll be PlayStation exclusives until at least fall 2018. But don't assume you'll actually get it then; the first Destiny's PS timed exclusives were until at least fall 2016, and halfway through 2017 we still haven't gotten pretty much any of it.
You will get them next fall. The taken King ones were meant to be until d2 came out sept 2016, you're only waiting longer because d2 never arrived.
Yeah, I wish that they'd at least give us the rest of it in D1 considering we're at the Age of Triumph.