The reason is that the adjective ruskie is typically understood as "Russian" in general parlance.
I’ve also noticed “pierogi kresowe” which is a reference to Kresy, a historical Polish region that includes Ruthenia, the origin of pierogi ruskie. I don’t recall ever seeing these before, so my guess is that the pierogi industry decided that Kresowe was a less controversial name to use.
The other issue is that people don't know how to translate "ruski" to English, so they would say "Russian dumplings", but in reality they should be called "Ruthenian dumplings" from Ruthenian Voivodeship.
They probably seen tourists turn their nose on "Russian dumplings", so they renamed them to "Ukrainian".
Since most of the Ruthenian Voivodeship is now in Ukraine it somewhat makes sense, but it is not entirely correct. Just shows lazy thinking.
In the UK, those sold in supermarkets are no longer called "ruskie" but simply potato and cheese dumplings.
When it's used as a noun it's pejorative for Russian people. When it's used as an adjective - it's neutral (and can be used both for russian and ruthenian stuff).
There's also more specific words - rosyjskie for russian and rusińskie for ruthenian, but very few people use "rusińskie" nowadays.
BTW there's this joke from communist times:
A waitress in a milk bar shouts "who ordered ruskie", someone in the bar shouts back "nobody, they came on their own".
There was a joke which used that, during the Communist Poland era (where country was thoroughly dominated by it's USSR neighbor and supposed friend, as per the official propaganda):
"
(in a bar)
- Who ordered russkie?
- No one. They came on their own.
"
The subject has popped up here and there. But more as a whimsical joke amongst people around me with more of an intelligentsia background.
There are however, many new places with Ukrainian pelmieni opening up everywhere you look. But that’s a separate thing.
I’m not a native speaker, but I always heard that the Greek derived Rosyjski/Rosjanin/Rosjanka were for Russians (ie the ethnic majority in the Russian Federation) but Ruskie was still reserved for the Rus’ in general.
I’ve had native speaking members explain the distinction to me, that ruskie is more of a catch-all term for the Rus’ peoples. Then again, my family hails from northeast and southeast Poland near the borders with the Rus’.
Is there a tendency to group them together? As Russians or as Rus’?
There is a lot of cultural context that Western Europeans (inc myself) are not aware of.