the brain 'can't process the numbers'
No that isn't what they said. They said:
it seems, isn't super great at processing actual numbers quickly.
Now I have problems with that comment because it seems to imply a few things:
This was THE reason for the change to analog
That the brain is sluggish with numbers
That there weren't a host of other factors, some you laid out, and some more important.
Not acknowledging where analog was non-intuitive
But the fact that a dial is on the surface easier to read than a bunch of numbers? Yes. Now are you sitting there for 10s reading numbers? Of course not. But you are instances faster perceiving shapes and images vs numbers - a series of shapes.
Though again OP missed out on 4. A lot of analog back in the day depended on consistent design. It is much easier to memorize a shape if all the speed limits are 15 30 45 60. It is a lot harder when the entire road system is hodged podged together with tons or rules upon rules where you need the accuracy of a digital.
Though again with analog a speedometer dial is probably looking neater than digital. If you think about showing progress of going faster or slower, you have a smooth arrow going smoothly up or down. The brain is good at interpreting smooth movement.
Think of a number going from 6 to 7. The animation of it. Think of how it flickers as lights turn on and off. You aren't stunned by that animation obviously but similar to say a game losing a bunch of frames you'll register that flicker. Even if for a moment.
Again that isn't enough to cause a crash and stuff we're talking about is micro-seconds. You don't need micro-seconds level of fidelity in your speedometer animation since the use case where that matters is a crash and you've probably to the point where it doesn't matter.
That said if you look at newer displays you'll note they use more smoothing, they start to use colors, different fonts etc. Even if practically it doesn't make an actual difference in driving, "it looks aesthetically more pleasing" and I think OP is confusing THAT for actual utility.
Stuff like this is in tons of communication and animation design classes.