If those prebuilts are desktop sized, I don't think they count as being the same. The form factor has a price associated with it, and has for a long time. If you're willing to let the size drift up you can do better... which is exactly what I did. I bought a prebuilt a few months ago when it became clear I didn't like the Steam Machine's value proposition, and put Bazzite on it. It has a much better bang-for-the-buck ratio partially because I have a place to drop a full-sized desktop into my setup without it bothering anyone, and much better upgradability. I might upgrade to the latest AMD later for VR if it ends up helping.

Which I suppose I'd add to point out I'm not a "Steam Machine partisan"... I was interested but bailed out about 3 months ago, before the price was even announced.

As for HDMI CEC, the machine I bought has an AMD 9060XT in it, and HDMI CEC seems to work fine. I didn't do much to make it happen. Maybe I flipped a switch to turn it on in the UI, but that was it. I've had a couple of people ask me why it works and all I can say is I don't know, because I didn't do much (if anything) to turn it on.