There was a Mantle thread earlier and it seemed to be very positive, like it was a good thing, so now it's just marketing?
Depends who you ask, heh. I think most folks are just curious to see what it actually ends up bringing to the table, though I'd imagine nVidia fans will prefer it to ultimately end up irrelevant.
As to how all the news fits into place, there's just way too much dust being kicked into the air to get a clear picture. MS
could be perfectly cool with Mantle as an additional API for Windows to help market games on the PC. On the flip side, they could be opposed if SteamOS heavily leverages Mantle and Steambox ends up taking off thanks in part to something like that. Or some other combination of factors.
Main point is, there's a ton of questions in the air with these developments. How everything will actually play out has to remain to be seen, really.
wouldn't that make the XB1 the lead platform for multiplats?
Not in itself, no. It would just make porting an Xbone game to Mantle on the PC easier, but devs would still have to work out a DirectX version as well for full PC support. What this does mean, in part, is that PC gamers with AMD hardware can looks forward to possibly better optimized software for their systems.
"Lead platform" will still mostly be up to the individual developers.