Trausti literally linked to the the Maryland attorney General statement on the case, where the defense attorney admits to his ms13 affiliation.
Here is a link to the house judiciary report. If you want more info you can simply search the victims name.
This isn't skepticism. There is no counter argument, no contrary opinion, no basis for the idea. No one is claiming he was not affiliated with ms13, not even the defendant himself.
Same reason I asked what level of evidence would sway you in regards to youth membership. I could link court cases. I could link the fbi reports on them targetting minors for recruitment. But I see no reason to bother if you will just dismiss it without a basis for why.