I have a M5 with a 223 barrel on it that I have never had a problem with. I use the Accurate mags and it feeds little short prairie dog bullets up to the 75gr amax, which I'm quickly running out of. I would imagine it would feed anything else just as well. I think the note on the website may be outdated. I remember when I bought my first M5 about 2013/4 time frame asking Ted about a 223 bolt head and he wasn't confident in how well they would work , so he wasn't planning on making one then. But the one I have, I've been running for a couple of years and I literally can't remember a single feed or extraction issue. Hit me up if you have any more questions or want to see pics or something.
I think I remember him saying it would probably be fine but would most likely be a push feed due to the geometry of the bolt head vs 223 case head. Can you confirm if yours is capturing the round or is it acting as a push feed?