I'm afraid you are, Svidgen, and I will demonstrate that point. Here are two pages in XHTML5 with nearly identical coding:
The only difference is that the first uses ° and — for character references, the second uses the easier-to-remember ° and —. Guess which page actually works and which gives you an XML parsing error.
The issue I take with some developers is that they think that, with HTML5, the doctype has become vestigial and unneeded. But it actually has a very real function when it comes to XML--which is REQUIRED when you want to mix HTML5 with any other language, such as SVG.