Yet you use it.
> I lean towards
Maybe because you just want to use it? Just look at the map[0], read the legend. Explain who were the people in 18, 17, 11 regions?
That's 14 years before 1933, and the border was not there by then, but...
I would prefer a slighly more credible resource than a postcard made in times of great turmoil.
https://upload.wikimedia.org/wikipedia/commons/c/cb/Evropays...
https://upload.wikimedia.org/wikipedia/commons/thumb/2/2f/Su...
There was no border in those territories for so long, anything they could have drawn wouldn't really have any meaningful historical connection. So IIRC they went by the Russian Imperial demographic maps and claimed most territories on those showed >25% "Malorossians". Ironically, this ended up pilfering some territories from Belarus, as well - note where Pinsk is on that map!
But, of course, this possibility does not exclude the other. If this was the primary motivation, however, it neatly explains why some southern Russian regions were also severely affected despite not having any separatist tendencies.