acrobat.adobe.com
That court case seems to refute him being a member of ms-13 that's why I said I've seen conflicting reports
I tried to absorb the legal document, but frankly it's confusing. Everything in the linked document seemed to revolve around complaints from those representing the defendant that there was no "proof" that he was a gang member. The whole thing seems to revolve around the fact that a confidential informant identified him as a gang member/associate. Evidently the immigration judge found that sufficient. The appellate judge evidently found it sufficient.
The issue appears to be that they screwed up in deporting him specifically to El Salvador. There was an immigration judge's order that stated that they couldn't. It looks like they could've deported him to absolutely anyplace other than El Salvador.
Now that he has been deported to El Salvador, the judge has issued a confusing ambiguous order that the US Government needs to put forth some level of effort to try to get him returned to the US.
I do think it's likely that if Trump really pressed for his return, El Salvador would comply. But how can you prove that?