The Javadoc should specify the right unit that the method actually returns.
I don't think they are EMUs though, are they? From a quick check, the return looks to be in 20th points, just as with HSSF, and as described at https://poi.apache.org/apidocs/org/apache/poi/ss/usermodel/Font.html#getFontHeight%28%29 I'd propose replacing the XSSFFont javadoc with the Font or HSSFFont one, would that solve the issue for you?
That's probably best. Looking at the source, getFontHeightInPoints has nothing but "@see #getFontHeight" which is rather inadequate documentation. HSSFFont documentation actually exists :)
Fixed in r1695528, and site republished with the updates in, thanks for your advice!