They changed that.
In previous Steam Hardware Survey it was listed as "AMD Custom 0405" but it changed now to "AMD VANGOGH".
The Steam data is what it is.
But let's show you that.
"SteamOS Holo" 64 bit: 42.07% (+2.74%) of all Linux
AMD VANGOGH: 42.05% (+2.73%) of all Linux
There a very small of Linux user that installed Steam OS in machines that are not Steam Deck.
But overall the numbers of Linux Steam Deck machines matches with the number of Linux SteamOS machines.
Only Steam Deck uses AMD VANGOGH.
Let just do simple math here. 2.5m of Linux monthly active users, 42% of them should be using Steam Deck, so it is 1 million users.
Steam Deck sold 1.6 million unit so far as we speak, some currently inactive, some installed Windows.
So your argument is invalid.