Cambodian is not a cuisine that comes up for most people during the "what to have for dinner" discussion, so it's not surprising that this restaurant is still a "hidden" gem (though it does always seem to have customers, which makes me happy).
I'm not sure why Cambodian food hadn't really caught on. If you consider where it is on a map (or discretely pull up Google to learn where it is on a map), you'll see that it's wedged between two countries whose cuisines have done quite well for themselves (Thailand and Vietnam). Cambodian food has similarities to both Thai and Vietnamese food, as well as its own signature dishes.
Now, I'm no Cambodian food expert (contrary to what I may have previously led you to believe) but I have enjoyed everything I've had from Reathrey Sekong so I think it's probably a good place to get it. Their basil seed drink is something everyone should try. I've also enjoyed their beef noodle soup (I don't think they call it pho but it's similar with a bit more garlic) especially the large amount of tripe. I plan on eating my way through this entire menu, especially the desserts, and I doubt I'll ever be disappointed.
Finally, I enjoy a good owner response and Reathrey Sekong really delivers on that front so bonus points for character.