That’s not only your opinion but basically also backed by the game files.
There are a couple of text files inside AoE2DE\resources\en\strings\key-value:
key-value-strings = Base game texts + campaign texts of everything up to Lords of the West. Also contains leftovers from the orginal game
key-value-modded-strings = Used for mods. If you e.g. want to replace a text from the regular string file, you can copy-paste the exact string ID here
PDLC2 = Dawn of the Dukes campaign texts
PDLC3 = Dynasties of India campaign texts
PDLC4 = Mountain Royals campaign texts
PDLC6 = Victors & Vanquished campaign texts
The texts for Return of Rome are stored in AoE2DE\modes\Pompeii\resources\en\strings\key-value
So the DLC must be in development for quite a while now and V&V was just a side project.