I’m fairly liberal on a lot of things myself, but it really bothers me when people think being “fundamentalist” is a bad thing (in all contexts). All religions are dogmatic and fundamentalist. Judaism is a religion. When you remove the dogma and fundamentalism, you have killed the religion. Deal with it.
And, of course, the great rebdoniel has the authority to Pasken by whatever Rishon he wants on whatever issue, even if that opinion has been roundly rejected by all the Poskim between then and now. It makes sense, after all. That is the Halachic process at Machon Hadar. Find a Shittah that says what you like (or something close to it) and say that that’s the Halachah. That’s how Ethan Tucker justified counting women for a Minyan. He had 1 Mareh Makom supporting him (he dug up a Mor Uktziah) and ignored everyone else. So rd is just doing the same as his Rebbe.