No true Scotsman would believe them, either. 
There’s no value for the Russians in that propaganda. No loyal American will believe them.
Given that we haven't declared war against either belligerent, why should an American's loyalty to their country be called into question if they happen to believe the Russian Ministry of Defense's take on a particular event over that of, say, the Ukrainian Ministry of Defense?
No true Scotsman would believe them, either.
“There’s no value for the Russians in that propaganda. No loyal American will believe them.”
Now you’ve gone from stupid to out right fascistic. The issue is whether it’s true, not “loyalty”. None of us (including you) know “for sure” whether the Russians have taken out 4 systems or have had one sold to them. The Zelensky regime would certainly not want to disclose it has lost any (especially losing one through corruption) while it is rattling it beggar’s cup for more. Beyond that, why would anyone assume that any battlefield piece of equipment can’t be destroyed?
In this case the RF has air superiority, which means it has a lot of drones searching for howitzers, MLRS, tanks, etc. When these things are found, the RF certainly has the means to destroy them. The strength of HIMARS is that they are deadly accurate and have some range. It’s a very good weapon system, but it isn’t somehow magically invulnerable to a rocket or howitzer strike. I’ve seen some of the video, and in the cases I saw, it was definitely an MLRS system that was destroyed. Of course, being an MLRS system doesn’t necessarily mean it’s a HIMARS. That would have been a sounder point for you to make.
As for being credible, the Zelensky regime, with help from the US/NATO and their compliant toadies in the press have published lie after lie in the press - garbage taken from video games, movies, made up out of whole cloth, etc. What I see on the RF side has been more consistently factually accurate, although I am sure not all of it has been correct. Yet, you and the other cheerleaders for team Biden/NATO/WEF simply ignore the unreliability of what NATO governments feed the tech media and the MSM and declare ex cathedra that any thing that suits your preferred narrative is true and that what doesn’t fit it is false. How very postmodern of you.
The Biden Administration has consistently excalated this conflict. The HIMARS has been particularly provocative because it’s clear the UFA isn’t operating them (the UFA also probably isn’t operating the M 777, Caesar Howitzers (there are photos of those that have been sold), and other advanced foreign weapons systems), and the intelligence for targeting HIMARS is coming from our satellites.
If, as you cheerleaders think, the UFA is a step away from finally vanquishing the “beleaguered” RF army, the RF’s economy is on the brink of collapse, and the Zelensky government is a wonderfully functional, reliable partner, why are stories popping up about sanctions being reversed or diluted and about NATO members becoming more ambivalent about providing weapons? Why is Zelensky carrying out massive purges in his iteration of the COUP government? Perhaps it suggests that the US/NATO proxy war is not going all that well, and the Zelensky government is not the heroic embodiment of “democracy” that we have been told it is . Even you can find those stories.