To be honest, the lightbox (which is a third party library) doesn't have great support for RTL, but I think there could be something else at play here related to CSS caching.
For example if I switch to RTL it breaks, but then if I do something which rebuilds the CSS cache, then it seems to work better... not entirely convinced it's doing the correct things in terms of the directions things move etc. but it certainly it's "less broken" in that things don't disappear off the screen.
Sometimes I can switch back to LTR and now LTR is broken.
So, I think the lightbox itself is working correctly, but perhaps something is off with our CSS cache.
He didn't suggest writing code, he suggested to make a change to the template, for the whole sake of having it changed, so you can do anything you want there, from a single letter to real code to a poem. All that's necessary is something being added or deleted, so XF will mark the template as “customized“. After that, you hit revert and the RTL error should be fixed - and your changes reverted, obviously.