I haven't worked with them, but the site looks pretty legit to me. Take this with a grain of salt as I haven't worked with them, but after looking around their site, it just looks like a Japanese-made site that is getting into original webcomics.
It's worth noting that a lot of things that you might have become very familiar with are only now really "hitting" in Japan, online. It's also worth noting that manga just means comics. Manga is not actually a style, it's a medium, and even if some outside of Japan may use it to refer to a style with predominant elements and influences, it's no different than someone saying "comics" or "sequential art".
As always, it's best to be cautious and read over everything in detail. I'd advise talking to them more and asking for any sort of contract you'll be expected to agree to, and what their terms in detail are (although the terms you can get on the site are pretty detailed).