It is, but it'n ot adviseable. The saliva coating your tongue should protect the tissue, but if you think about what you are doing, and your mouth goes dry, your tongue can stick to the poker, which could be a little unpleasant(!)
I just wondered - I wasn't thinking of trying it! I thought it a little odd that he said it had to be white hot - if it was only red hot it would take your tongue off.