Yeah it's a PR thing. I mean, Alonso had no interest outside F1 until he could suddenly offset his lack of F1 success with this new found glory.
But that said, I think there is an impressiveness is being able to win the 500(one of THE big oval races), Le Mans(arguably THE big enduro, vs Nurburgring 24 and etc), and Monaco is a good one if you had to pick a single sprint road race rather than a league. Mainly because it's a street circuit. But part of what makes Indycar so fun is they do ovals roads and streets in the same year so...
Maybe more so now due to how specialist everything is. On the other hand race cars and racing have kind of narrowed in their formats, Le Mans cars are just single seaters with more bodywork, you don't really 'manage' races in the same way, etc.
A modern true Triple Crown, in more of a best of the best sense, would be any topline pavement racing (NASCAR, Indycar, F1) wins of note, and a WRC/Baja/etc, and a motorcycle result. That'd be diversity. Hell an F1 driver having success in NASCAR would be more newsworthy and a greater accomplishment than winning in an Indycar or sportscar prototype or other 'family tree' form of racing. Nelson Piquet Jr for all his faults has wins in major NASCAR touring series, European single seaters, Formula E, some sportscar racing possibly, whatever road-course 'stock cars' he's racing in Brazil currently, etc, etc, et al.
What was weird to me with all this ALONSO ATTEMPTS TRIPLE CROWN EXCLUSIVE FILM AT 11 hot air, no one ever ever ever talked about Montoya. Indy in 2000, Monaco in 2003(?), why none of the prototype entries took a gamble on him in the time since was surprising. He has a much much much better chance of cherrypicking a Le Mans win than Alonso does at Indy.