I see nothing wrong with giving a player a look at another position to see if that's a better spot to make an impact. He has size, but was very raw when he got here and probably hasn't developed at OL as they thought. However, because of the roster exemptions for him they kept him around. There's a low probability of making the team anyway, but
I think an important aspect of the original tweet is that the team already has seen him at DT some in practice and liked what it saw enough to give him a shot. Doesn't mean he'll succeed with the transition, but it isn't like they went, "He's not going to make it as an offensive lineman, so let's give him a shot on defense." A lot of the time, practice squad players and bottom-of-the-roster players have to play multiple positions on scout team to simulate the other team. Apparently, Alarcon showed a little something.
Of course, a few years back that project TE we drafted (the name escapes me) apparently looked like a world-beater catching passes from Romo on scout team and then just disappeared. So I won't be surprised if this experiment fails. However, how many exempt years do these international players get? If he stays exempt, then might as well keep him for scout team purposes.