2. (1) Subject to section 4, where a maximum rate of speed other than that prescribed by subsection 128 (1) of the Act is prescribed for a highway in a local municipality or built-up area, speed limit signs shall be erected on the highway, in each direction of travel,

(a) not more than 600 metres apart where the speed limit prescribed is 60 kilometres per hour or less; and

(b) not more than 900 metres apart where the speed limit prescribed is greater than 60 kilometres per hour and not more than 70 kilometres per hour. R.R.O. 1990, Reg. 615, s. 2 (1); O. Reg. 175/08, s. 1.

(2) Where the maximum rate of speed for a highway in a built-up area more than 1,500 metres in length is that prescribed by subsection 128 (1) of the Act, speed limit signs shall be erected on the highway not more than 900 metres apart. R.R.O. 1990, Reg. 615, s. 2 (2).

"Where the maximum rate of speed for a highway in a built-up area more than 1,500 metres in length is that prescribed by subsection 128 (1) of the Act..."

Specifically the "is that prescribed by..." --- isn't the speed prescribed in 128(1) basically the 50/80 unless otherwise posted thing?

You may be over thinking this. It basically states that "if" the prescribed speed of 50 in a municipality or 80 in a rural ares are deviated from, then the speeds designated must be posted within those measurements.

There is still no requirement to post 50 or 80 zones in their correct areas.