The Russian regime (and apparently a lot of Russians) deem Ukrainians as an inferior ethnic group - they call them "little Russians".
Ukrainian authorship would mean:
- Ukrainians are competent people with agency (which they are of course, for lots of reasons) - this plays into ethnophobia;
- their government, military, etc, is competent, functional with agency - this plays into government legitimacy;
- Overall, in a lot of instances, the Russian government is incompetent, even more incompetent than the guys their propaganda has been trying to paint as corrupt, incompetent people who are being manipulated.
That's why a lot of time Russian propaganda trys to spin Ukrainian wins as "NATO/CIA/MI6/external agent did this".
For example, they tried really hard to bend reality to remove the credit for the Ukrainian drone operation that destroyed a lot of bomber jets, saying it was planned and executed by CIA, MI6, Israel, etc [0].
This is what we're dealing with here: massive ethnophobia and propaganda.
So in their propaganda, Ukraine can't be competent and stand on its merit, because that would mean they're not inferior people and that they have agency.
You should always be wary of someone making these claims without any evidence.
[0]https://uacrisis.org/en/rospropaganda-zaplutalas-v-pavutyni