I've tried to figure out how she got this information. Bellingcat/Aric Toler said that the NYT were the first to publish an article referring to the OK.ru profile. Bellingcat also sourced Karol Markowicz as the first one to share his specific birthdate.

Is there a publicly available database of ex-cons with that information that can be searched by name and city?

Did police in Allen give it to her? If so, why wouldn't she specify that's where she got it from?

That's how the profile and everything on it was allegedly found. So how was his birthdate found. We're told that he had no online presence other than the OK.ru profile.

I've replied to her on Twitter, replied to Aric Toler asking the same to see if he responds.

This is the key.

If there's no good answer to this then it means that law enforcement leaked it to her and she chose not to disclose where she got it from.

Maybe I'm not aware of some database that she was able to find it on or I'm missing something else but seems like police haven't released it publicly and the whole thing is based on this being his birthdate.