Him and the Golden Knights are circumventing the cap by having him on LTIR and waiting out his contract. I doubt he's even rehabbing anything and he won't play for the Vegas Golden Knights again.
Half the players in the league above 30 could probably qualify for LTIRetirement due to some sort of long term issue that would be evident enough for them to be deemed eligible by a physician, but can be willingly played through with discomfort.
It's pretty obvious that so long as players and teams have the same priority, LTIR can be worked around. Vegas aren't the only team who does it. Look at Clarke MacArthur coming back in Ottawa when I assume he determined one last shot at the cup was worth whatever risk he was taking, and then after still being able to play at a reasonable level, went back into LTIRetirement after the Sens run. Look at Hossa choosing to not play through his dermatology issue when he no longer has incentive to do so (salary dropping, Chicago's window getting smaller, etc).
I don't know about Lehner's situation, but I'm just pointing out that if both a team and player would rather the players not play, most players above 30 probably have an issue that would qualify them for LTIRetirement.