I think the point that was being made is that BD releases cannot be SD-only. That's not to say that they can't include any SD content, but that a BD release must contain some HD content.
This is what makes it a licensing issue, rather than technical. BD, at its core, is just a storage medium just like a CD, DVD or even magnetic tape. So, BD is capable of storing pretty much any digital content you'd like, but to license BD for a home video release it must contain HD content.
I still don't see any evidence of this. There is not, IIRC, an actual legal requirement that all BDs must contain at least some HD content. It's not enshrined in law or contracts or anything like that.
Now of course it would probably just be good business sense to have all BRs have some HD in them, because customers would be confused if they didn't. People *expect* Blu-Rays to be in HD. But there is no legal, formal requirement that they do. If a company wanted to release a BD that had only SD content, they could. It would probably be a bad idea, but they still could do it.