Crucially, "the 7-bit coded character set" is described on page 6 using only seven total bits (1-indexed, so don't get confused when you see b7 in the chart!).

There is an encoding mechanism to use 8 bits, but it's for storage on a type of magnetic tape, and even that still is silent on the 8th bit being repurposed. It's likely, given the lack of discussion about it, that it was for ergonomic or technical purposes related to the medium (8 is a power of 2) rather than for future extensibility.