@Hitch -- like @AppleTard says, calibre supports dual-MOBI just like Kindlegen.
When we say dual-MOBI, we actually mean it.

Similar output to Kindlegen, as in a file that has both KF8 and mobi7 sections, for compatibility with new and old devices.
calibre supports:
- AZW3 -- KF8
- MOBI -- mobi7
- MOBI -- dual-MOBI with mobi7 && KF8 sections
#3 is an (advanced) setting in Preferences ==> Conversion ==> Output Options ==> MOBI Output
Usually people don't bother to use it, as the only real reason to create a dual-MOBI is for KDP uploads

and you should be using kindlegen if you want Amazon to accept it, but someone recently (past year or so?) discovered that Amazon's PDOC service now accepts dual-MOBI in addition to old-MOBI.
Result: new devices download an AZW3-
formatted 
ebook, almost as though it were a "real" book.

i.e. sold in the Kindle store.
The old way (email a plain-old MOBI and get one back) works too.
As you say, it probably uses the same backend as KDP although it is
far more forgiving of calibre conversions, for unidentified reasons.

So it makes sense that the font issues in KDP would affect PDOCs as well.
Amazon has continued their trend of having a buggy backend. PDOCs that make use of this hacky route to get delivered as AZW3 don't download their covers. MOBI works fine of course.
Almost makes you ask WHY THE %#*@(&*^$ they decided all covers must be downloaded from their servers rather than extracted from the ebook file (at least as a freaking fallback

)!!!