I swear there was one but can’t seem to find it, am I just going senile?
Apparently, you do, as there never was a Submondo source book announced in the Kickstarter project or anywhere else.
And now, that every announced book from the Kickstarter project is produced and delivered, it is very unlikely that there will be any Infinity 2d20 product by Modiphius anymore. This was probably not a very successful product line beyond any pre-financed Kickstarter products by backers, hardly any sales beyond that, I suppose. And so there is no reason to put any effort in such a low-selling line.
That said, I would have liked a few more source books, especially a Submondo book.
I don’t know whether Corvus Belli has produced much material about Submondo subfactions for their miniatures wargame, as I am not into miniature wargaming, only roleplaying games. Maybe there you can find more information?