I get the sense that implementation efficiency was considered as part of the spec: if it wasn’t efficient, vendors with existing FP hardware would have less incentive to switch. Allowing operations to happen via simple bit twiddling (in software or hardware) would be part of that.
I mean, what else were they considering if not implementation efficiency? It's not hard to come up with some representation for floating point numbers if you don't have to be concerned with efficiency.
Also correctness. There were a lot of implementations already that had poor behavior at different points in the number space (especially with respect to very small numbers).
Kahan also had experience because of his work for Intel on FP coprocessing. He couldn’t talk about the then not-yet-released parts, but he pushed hard for implementation decisions that other committee members said were too expensive but that he knew had been solved at Intel.
