I worked on a compiler because that's what Corion suggested I do.

The distinction between compilation and interpretation is orthogonal to the ability to write a BNF (or another specification) of the language's syntax. Compilation versus interpretation is a detail of implementation.

What ikegami is trying to understand is if there are legitimate documents you cannot produce from aXML programs because they contain character sequences that could be aXML instructions. (Certainly you can modify the plugin environment to avoid that, but there's a tarpit to avoid in doing so. Whether that's a problem in practice is up to the user to discover, but I've worked around quoting issues in polylingual programs before and have no inclination to do so again.)