Perhaps he could have, but according to Walmsley's own account, he followed Benson's direction. He also said he appreciated what he learned from her about more speed/interval-based training, but at the same time would have done more if it had been up to him. Reading between the lines, one gets the idea he felt he didn't run to his potential in college (though I don't want to put words in his mouth) due to the reduced mileage.
I agree in general, though one spoiler may be how hot race day is in Atlanta. Walmsley is an exceedingly good heat runner, as both his 2016 (aborted) and 2018 (record-setting) Western States runs showed, where temperatures were as high as 95 to 100 degrees over the latter half of the race. If temperatures are brutal, that could help even the playing field for him in terms of hanging closer to the lead pack. To me, this possibility, especially combined with a hillier-than-usual course, is what would make his attempt against the other guys really interesting.
Of course, Rupp is good in the heat as well, and Ward seems to do well in it also. I would venture Walmsley is better in the heat than they are, but not enough better he's likely to challenge. Walmsley vs. the other guys is where it could get the most interesting on a hot day. If the weather turns out cool, his chances go way down.