You’re correct, and you’ve figured out the “correct” way of clearing it.
I remember reading about this very issue in another brand (ecoboost engines). You can regenerate the filter two ways, or rather, there’s two options – one of which relies on the car itself doing its job.
The first one is as Kia describes, drive forever and hope that the car puts the engine into a “lean state” by altering timing. It’s supposed to do that, but clearly it doesn’t really work.
The second (yours) solution is generating the same effect, except you’re not relying on the car to alter timing. By decelerating through engine braking, you increase the oxygen in engine and exhaust – the same outcome as changing timing to run lean. It only tries to run lean to increase oxygen flow through the exhaust, which increases the temperature inside.
You can absolutely get that effect by accelerating to 75mph, and dropping down to 50mph through engine braking repeatedly (from high revs).
If you don’t have hills to coast off of, just accelerate to 75mph in third/fourth gear (whatever leaves you at around 5500-6500rpm at 75mph), and then engine brake down to 50mph multiple times. Probably best done at night, would be my suggestion.
That all said, this is a solution to something that shouldn’t be a problem in the first place.
Prior: SEAT Ateca Xcellence Lux 1.5 TSI DSG MY19, VW Golf GTE PHEV DSG MY23
Current: Hyundai Ioniq 6 Ultimate
Next: we'll see what's available in 2028.