But it wasn't just fixed "flat"... The angle of the gun along the engine wasn't flat to your path of motion and Bf109s still harmonized the gunsight so that it was actually between 3 and 5 degrees upward to the line of the engine itself. Remember when you're talking distances of 250 yards (700+ feet), a VERY tiny half a degree had a large impact in final impact of the round.
Further, even if the gun is rigidly fixed, your viewpoint relative to the nose of the craft changes where that bullet/shell impacts. So even IF it were 100% fixed all you need to do is adjust your pipper on the reflective gunsight and you can still adjust your convergence -- effectively -- without touching the gun at all.
Germans were rather fastidious about their standards and practices and they could and did make adjustments to such things through one means or another.
What does it matter HOW it was done? The end result in AH is that you are pulling off the same effect.
Bf109Fs, from what I remember reading, were harmonized at 400m, but on the upward lob of the trajectory would also pass through the aimpoint at 200 yards, and German studies (unrelated topic) were noting the most effective ranges were 200m or less.
The fixed convergence and harmonization wasn't that different from US or RAF convergences -- it was a standard and the pilots were forced to fly it until that standard was changed or unless somebody had enough clout to break the standard. I have read anecdotal accounts of Luftwaffe aces that had custom convergences, but for the most part the fixed harmonization/convergence was for all the green pilots with no skill that couldn't hit the broadside of a barn. It didn't mean you COULDN'T change it, it meant they were anal retentive and didn't allow it in most cases.
As a counter-argument: RAF pilots in 1939/1940/1941 were hampered by the wide spread -- a 12-foot rectangle so that inexperienced pilots would hit at least with SOME bullets before they lost their shot or ran out of ammo. The problem was that the pilots who could actually aim were frustrated and hampered by this and it wasn't concentrating fire on a single point -- that came much later after much complaining and reports of better ways from other services. It meant that many Luftwaffe bombers got away with scattered hits and no real damage when otherwise a good shot would have downed them. Now imagine in AH if the Hurricanes had fixed convergences. It would be 10x worse than you think it is now!
This is really a non-issue. It's taking the information and drawing the wrong conclusions from it.
Information: The tube in a 109 engine had little to no room for harmonization, so it should be fixed flat and drop as soon as it comes out of the nose in this game. Well it doesn't take into account other factors, the fact that all you needed was a fraction of a degree, the fact that you could still tweak the gunsight, the tales that some aces did, and the fact that the prescribed convergence was no different than other nations' prescribed/dictated convergences -- it was a rule, not a physical limitation.
Hence, IMO, the topic of the hub guns not being converged is a non-starter and not worth all the effort put into it so far.