Interesting that it uses a dedicated 9070 (ugh that name), rather than being a custom APU, like the other AMD consoles and the deck. That just adds cost and design complexity. You've got to package multiple chips, have separate power circuitry for both, have separate memory pools, and a more complex cooling system.
That said, perhaps they're being cautious - if you can't sell a lot of them, a custom APU isn't worth it. It's also probably much faster to bring to market if you're using off-the-shelf parts.
E: the source appears to only say Valve is working on 9070 drivers, therefore a steam machine must use a 9070. That's a bit of a logic jump. For starters, any driver work on the 9070 will improve any other RDNA4 chips (including APUs). Also, Valve has done driver work on plenty of things that they don't use in their own hardware, from various AMD cards, Intel graphics, and even work on the open source Nvidia drivers - they are gearing up to a general release of SteamOS, after all.
TL;DR: this rumour is almost certainly bullshit.