It looks like both you and Ron Pattinson are correct about the additional sugars being allowed in top-fermented beer, not bottom-fermented beer.
Here's the link to the actual document Ron is referring to in his piece that you linked (the text is from The Provisional Beer Act of 1993
But the original text from 1906 basically states the same thing here:https://de.wikisource.org/wiki/Bekanntmachung,_betreffend_die_Fassung_des_Brausteuergesetzes
Unfortunately, it seems that Horst Dornbush and Karl-Ullrich Heyse's entry in The Oxford Companion to Beer
(pp. 692-693) is incorrect. Here's what they wrote: "The German version [of the Reinheitsgebot], on the other hand, is slightly more lenient when it comes to bottom-fermented beers. These may also be made with the addition of "technically pure cane sugar, beet sugar, invert sugar, and modified starch sugar, as well as coloring agents made from these sugars.""