It isn't a loose proof.
Didn't want to call or but I didn't got an exact word. It am not much convinced with this.
You have already said that it is a mirror site , which means it just mirrors the bitcointalk.org posts. The post in bitcointopics.xyz is legit.
I might call it as a mirror site but others find posts inside forum much more convincing and I follow the same while giving feedback. DT members or anyone don't usually gives feedback based on things flying outside forum . Possible that the bitcointopics post might be edited. Also it might be that BlackMambaPH might not have posted it.
The source of the post in bitcointopics.xyz sends to an archive.is link which also doesn't shows me any BlackMambaPH's post . However I am still trying to check via other mirror sites.
Rollin also confirms the evidence and it is a substantial proof.
What rollin has confirmed is that he had followed giveaway rules and his account was credited.There was no quote of the BlackMambaPH's post with his Twitter account and that rollin address by Rollin.io
What people in DT and community is believe is not to give a harm to someone reputation until they are entirely convinced. Believe me , they wouldn't until they get entirely convinced and I respect the same since it puts a question mark over someone's reputation which might make the concerned user lose his customers or whoever who was willing to trade with the user.