Does Steam prevent crossplay in some way or is he just throwing a name out there without any particular backing?
The latter per usual. Activision used Windows Stores and Steam's built in servers instead of making their own crossplatform ones because it was cheaper and easier. But Tim continues to show his ass on topics he knows nothing about.
Steam's new Online API is fully crossplay and compatible with other storefronts if developers want to. This was a big selling point in their GDC conference.