Total urban myth.
One I used to believe & I’m always advocating being in the right gear at the right time when you need to open the throttle again. Engine braking does have its part whilst normal riding, but we were talking about emergency braking.
If you are braking at the limit then the front wheel is just off locking, the rear wheel is just off locking, then extra engine braking would just lock the rear wheel. The throttle is shut so there is no real power, certainly nothing that could be effective against the power of the rear brake. So how can the ‘compression’ help?
When I’m racing, depending on the corner I may go down 3 gears at a time & the clutch is in, yes I’m feeding it back in later so I have a stable platform & so I can have some driving force to keep the weight off the front wheel at the apex, but the point is these are hairpin corners at max braking & the clutch is in for a reasonable part of it.
Maybe previously you had an older Guzzi where the brakes consisted of 2 cinder blocks rubbing against a piece of wood & any assistance from the engine was welcome.
(That should get the purists on my case).

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