Storage-gating is really disgusting considering how much Apple charges for storage.
So this made at least some sense.
I guess yields might be good enough that they can afford to bin with another core in there as well.
Memory is probably still the main reason for binning in the first place.
For example: https://forums.macrumors.com/threads/m4-mbp-ssd-speeds.24422....
Compare with: https://www.tomshardware.com/pc-components/ssds/crucial-t710....
While it is not an Apple to Oranges comparison, T710 seems 80% faster for writing big files, and for $279.99 - $299.99 for 2TB this is still much cheaper than whatever Apple is offering.
If you have a better reference (specially if there is something that is cross platform), I would be interested.