No, the upper byte of key should have 256 possible values. It now has one (all zeroes, essentially). Keyspace is reduced to 1/256 of original, a reduction of 255/256.

Only for that byte -- the overall keyspace is 184 bits instead of 192 bits.

I think his point is that 2 raised to 192 is 256 times larger than 2 raised to 184, meaning that you are getting 1/256 as many possible keys by reducing the key length by 8 bits.

Ahh, that's a good point. Thanks for clarifying.