I've bought some odds and ends from Add Helium in the past. I'm not a rebreather diver, mind you, they just had a valve part or something in stock that I needed.
I've seen composite cylinders for sale on their web site in the past. It always seemed abundantly clear, to me, that the composite cylinders they were selling were not manufactured in compliance with DOT regulations. I don't think there are any composite cylinders approved by the DOT that are suitable for underwater use that are close to that size, so there's not much room for confusion.
Filling any HP cylinder involves trust. I tend to trust DOT stamps because they have a track record of trustworthiness, and because there are multiple engineering, procedural, and regulatory controls in place to make these cylinders safe.
Making a decision to extend that same level of trust to a manufacturer, importer, or retailer is something that rebreather divers do all the time -- if not with cylinders, then with the rebreathers themselves and the consumables that they use. It is one of the reasons I will probably never be a rebreather diver.
I think that for divers there are only two relevant questions this thing brings up.
1. Is there anybody out there who actually thought they were getting a USDOT approved composite cylinder for their rebreather, and who relied on that information in making their purchasing decision?
2. In the context of rebreather divers who perform their own fills and are willing to knowingly use non-DOT cylinders, do these pose an unacceptable risk?