Help with a translation

[D B Campbell]

But it isn't sure that the promoti indigenae sub thebaidos are linked to III diocletiana in ND.

Seeck's reading can be found (e.g.) in the internet versions of the ND, such as Intratext. As you say, it's the section on the Dux Thebaidos in Egypt.

But if you check Seeck's apparatus criticus, you'll see that he split a perfectly good entry into two:
Equites promoti indigenae | legionis tertiae diocletianoambos
To do this, he had to alter the second part to read: Legio tertia Diocletiana, Ombos
(Ambos or Ombos is the location of the unit.)

Lewin (also in addenda reports Brennan ND intepretation) examining the papyris and a inscription from diocletianic era, observes that probably indigenae are a title used from the start of IV century and that no link appears between the promoti indig. and a legion, but appears for the normal promoti.

Lewin's argument seems to be this: he cites an inscription from Bkhara which mentions equites promoti indigenae; he notes that they might have been detached from the nearby legio I Illyricorum (the "indigenous" legion); but concludes that, because the inscription doesn't say so, they couldn't have been.
This doesn't seem (imho) to be a very secure argument.