Because something is printed in RT doesn't make it untrue. That something - critical of Russia's enemies - is only reported in RT and not in any other major new source - is a strong contender for being untrue.

For something to an example of the ad-hominem fallacy, it has to be discarded purely on the basis of the claimant. If something is being rejected on the basis that the claimant is unreliable and contradicts generally reliable sources, that is something quite different.

Should I start discounting RTs weather report for NYC based on their political reporting feel free to call me out on fallacious thinking.