I used to be very wary of sites that were registered under a proxy, but after I got spammed, and called, and snail mailed to death in regard to my domain, I can see the advantage. I'll deal with a proxy registration as long as the site is open about contact info.
I'm realizing this as well, and I'm just doing a tiny little portfolio site. The spam has been just tremendous. Tastes like plastic.
It's nice to see a company reach out here in a professional manner
. I don't see any harm in following up with them about references if they're willing to provide. You should always check that sort of thing as far as you can--so search out the books they've worked on, read a couple, check the authors' own presence, see if there are acknowledgements in the books about the editing, etc.
Worth remembering as well that you actually need a freelance editor. There are loads of legit reasons, including and especially self-publishing (I think you mentioned you were doing this in another thread, OP?). If you're looking at the trade publishing route, though, hiring an editor is probably unnecessary unless you can't self-edit and revise for some reason.