Exactly! Very well said, it's what I was trying to explain.
Don't agree with your conclusion in the end though. IIRC they have fixed ratios all season which would prevent something like this from happening. I think they are somehow being much more aggressive in their deployment over a Q lap in the early part of straights and are somehow able to be at peak power for longer as a % of total laptime.
This isn't about gear ratios at all though. My little theory on what Ferrari could be doing is unrelated to the gear ratios or whether they can be changed during the season or not.
I'm sure most people are familiar with the term "engine map". What I'm talking about here is the inner workings of the engine map itself. For a given "engine map", it's basically a description of how to take a series of inputs and convert (map) them to a series of outputs. The outputs would be things like what mode the MGU-H and MGU-K would be in, the air/fuel mixture, the ignition timings and so on. One of the inputs would be the throttle pedal. I don't believe there's anything disallowing the currently select gear being part of the input either. In other words, in the same way that it's possible to have an overall Q3 "party mode" that is more aggressive than the Q1 or Q2 engine maps, I think it would be possible to have an engine map where some gears have more aggressive settings than others within a given engine map.
So for a specific engine map, you could have (say) gears 3, 4 and 5 using more aggressive settings than the other gears. This would give you an extra kick at the start of the acceleration zones, improving performance down the entire straight.
Why not use these more aggressive settings for all the gears? Because it puts too much strain on the engine. For gears 1-2, you would be traction limited anyway and wouldn't need more power. A F1 car will accelerate through gears 3-5 quite quickly (2 seconds or so) but down a long straight keeping the engine in this more aggressive setting for say 5-10 seconds would put a lot more strain on the engine, particularly at higher revs. The reason why "party mode" gets limited use is the same - it puts more straight on the engine, so the usage is limited to when it's most useful. For an F1 car accelerating out of a corner, the most useful time to have some extra power would be at the start of the acceleration zone - an extra 20 hp for the first second would be more useful than an extra 20 hp for the last second. It wouldn't be legal for the engine to automatically adjust the power based on its position down the straight but if you have different power settings for different gears then you can get pretty much the same benefit.
All the PUs probably have slight differences in the engine map for different gears. My little theory with Ferrari is that they've gone very fine grained on the optimisation between power and lifetime for the engine and taken this concept a lot further, particularly for qualifying. They've probably improved the base engine along with it as well.