I haven’t seen this R’ Moshe inside. Does he actually delineate where one may go, or is he just saying that since in theory there are areas that one may walk on it is not reasonable to assume that this is why the Tor/Shulchan Aruch would omit the issur.
It is possible to hold that there are parts of HH”B that one may walk on in theory, but if we aren’t sure precisely where they are it would remain assur in practice (safek issur di’Oraysa li’chumrah).
zogt_besser
Americans don’t allows automatically follow R’ Moshe, especially when he is a daas yochid. (For example, almost all Americans use shabbos clocks and listen to music.)