What is your source of information for transit without visa (TWOV) in Russia allowing pax to go landside?

Looking at Timatic (which is what most airlines will look at when deciding whether to let you board at your point of origin), TWOV is indeed available in Russia for 'holders of onward tickets for a max transit time of 24 hours'. However, the standard TWOV rules still apply to this, and these rules:

• Define TWOV as 'Passing through an international transit area of the airport in order to board a connecting (or to proceed by the same) flight, without entering the country (i.e. clearing immigration).'
• Require that 'unless stated otherwise, passengers wishing to TWOV must ... remain in the transit area (airside) or on the aircraft.'

(emphasis added)

Some countries do have exceptions that allow TWOV to include going landside - but I can't see anything in Timatic that would allow this for Russia. I haven't transited DME, but I have done international TWOV at SVO - and I can't see how you'd get on a domestic flight there, without going landside.

Of course, if Russia's TWOV rules did allow going landside, then it shouldn't be a problem to take a domestic flight in that time (unless the TWOV exception excluded that).

However, to take a domestic flight, I suspect you would need a transit visa (see VFS Global if you're in the UK; transit visas valid for 3 days.)