When the host OS is 64-bit, yes, it is. It is just as backwards compatible as a x64 CPU is to x86.
Jesus fucking Christ, did you read the damn thread? Claiming you can't provide backwards compatibility without a VM because of the processor is provably bullshit, because, as was already pointed out, Wine on 64-bit Linux on an x86-64 cpu can run 16-bit Windows programs without a VM. Yes, 16-bit Windows binaries running on an x86-64 CPU in 64 bit mode, no VM layer.
It's not that Microsoft could not provide real backwards compatibility. Hell, the Wine code is all LGPL, so there's nothing stopping them from taking the Wine code as the core of their own effort to provide the backwards compatibility. It's that Microsoft, deliberately, by choice, did not provide real backwards compatibility.