This sounds like an offhand Youtube comment, I'm afraid. Underrated how? Its principal strength, easy embedding with the ability to work as an extension language, is well known in the circles where it matters. The authors never gave an impression that they'd aim to make it a language to bury all other scripting languages, which I find refreshing in the winner-take-all culture of programming language discussion. Lua is modest and works well for what it is. No need to go all grandiose.
> I just wish one of the mainstream browsers actually puts their foot down and starts supporting Lua as scripting language.
I sincerely hope not, that would be a very counterproductive dilution of effort. Browser authors already have their plate full with all other web platform problems.