i honestly woulda thought that it would be smarter to launch the handheld first rather than ask people to buy two lumps of new hardware. that could be $1000+ of spend if they launch them together, they'll cannibalize and compete with each other.
I guess there'd also be development stuff to worry about there. Yeah we shouldn't expect generations to be prominent as in the past, but I don't imagine they'd want to unnecessarily confuse devs by releasing new hardware within a year or two running off a different architecture or baseline. Even on the consumer side a handheld right before the transition to the next hardware lineup (generation) would feel very strange. And then there'd be concerns about how long it could be developed for and how many games could hit it. I imagine that's already part of the concern as apparently this same interview said Xbox wants their users to access as many games across the Xbox ecosystem as possible, but in the grandscheme compared to its total library not that many Steam Deck games are Steam Deck verified. They could still separate their launches and give a handheld a couple years into next gen. That'd also be interesting as they could maybe compromise on power a little less. But I don't see a huge problem of launching them together. People will buy the console they want first and then the other down the line (probably on sale).
But it's all just speculation. I think as is Xbox's Play Anywhere and PC/Cloud efforts really help devices like the Legion Go fill the gap and perhaps that's what they mean. There's a lot they've already learned from the PC Handhelds for windows gaming. Heck, really the PC Handhelds have forced Microsoft to make changes that they should've a long time ago for gaming on windows. As they let that market grow, see how developers and consumers interact with it, and better yet let it evolve (as in the tech powering it), they can build a better handheld.