The speaker thinks the reward-is-enough idea becomes impractical because it may be unrealistic to find a suitable reward function and optimize it with available techniques.

“the question is more whether this is a realistic assumption to make like whether having like a god-given reward function which just does something is”

“Whether it's a realistic assumption,”

“even finding it and once we have it, whether it's realistically optimizable with any technique and that is where I think the the actual like the practicality or like the the reasonableness of this breaks down.”