I guess this thing is helarious for both of them, because it will turn out a funny situation to watch. Lol, they're not legit ufc fighters and they're just called to have match like popular Bieber and Mr. O whose been a cast of jackass movie. Although it seems awkward to see them doing that sport, I agreed that Steve-O can handle pains instead Justin Bieber because I've seen how Mr.O endure painful stunts in their jackass shooting scenes.
Justin will not accept this invitation because he knows he’ll get badly hurt if he decides to step in the ring, besides I think this is just a publicity stunt from Steve. As you have rightly said these two are are not real UFC fighters, and I don’t think UFC management will allow them to get in the ring and spoil their sports name. I wonder who else will throw his name now to fight against Justin, it seems many people want to fight with him.
The management of UFC, of course, Dana White agreed to hold this fight actually Dana wants Cruise and if Tom doesn't step up he needs a good replacement and Steve O volunteered for this fight and Dana gets a nod check this article to prove it and it's now on Bieber's hands if he agreed to fight Steve O
https://www.tmz.com/2019/06/27/steve-o-justin-bieber-tom-cruise-ufc-fight-dana-white/