This statement in 16.2.1 Pointing Elsewhere, is not true or potentially misleading, depending how broadly one reads "In general.":
"In general the current base URI in force is the value of the xml:base attribute of the closest ancestor that has one. If there is none, the base URI is that of the current document."
xml:base attributes accumulate through the hierarchy. The current base URI is not just closest ancestor OR base of the document. It is the concatenation of path segments, beginning at the top of the hierarchy or the nearest http:// (or ftp://, etc.) and accumulating down to the context node.
If "in general" means "in practice, it turns out that usually," maybe the statement isn't false, but I would take "in general" to mean something closer to "except for corner cases explained in the spec." And I'd be surprised if the sample set of @xml:base uses is large enough for the Guidelines to confidently say what is the norm.
Maybe not a big deal, but I was mislead by that binary assumption when I was first started learning @xml:base.