He specifically said "since it was all inside a single tag". Without knowing what the payload of the tag is, its hard to say if regex was appropriate or not.
I'd say it's never appropriate to parse XML with a regex. Even if you're just looking for something "inside a single tag", there are just too many gotchas like entities, CDATA sections, etc. Why bother with (or forget) any of that when there are well-tested XML libraries for probably every language, and many languages have it in their standard library.
I think you and I are reading the statement differently. I did not read it as he was using regex to parse the XML; I read it as he as using regex to parse the text contents of a particular tag that he extracted using a standard XML parser.
Of course regex is appropriate in some cases. You parse XML with a standard API, then use a regex on the contents of a single tag. For all we know, inside the tag could have been "img: foobar", regex works great in that case.