Someone got their sums wrong, the 88mm could range between 32 and 35,000ft.
Maybe Arnold thought so too, he relieved Eaker for Doolittle in 1943.
Doolittle took over in 1944, and the Eighth continued to bomb well below 30,000 feet.
I don't really understand why you can't envisage combat at 30,000ft+ just because the war didn't last long enough for that to happen.
Since the subject of Renrich's comments concerned what did happen vs. what might have happened had the war continued, I found his points to have merit.