Brachos 21b - Interruptions during the Amidah

According to Rabbi Yehudah and Rabbi Shim'on the students of Rabbi Yochanan, one is allowed to respond Y'hei Shmo HaGadol (Shmei Rabbah) Mevorach during the Shmoneh Esrei, as one is even allowed to respond YSh"R while engaged in Ma'asei Merkavah. (The gemara notes that we do not hold like them.)

Why is this a proof? Why would we think that one should not be able to respond YSh"R while engaged in Ma'aseh Merkavah - is it not merely an exceptionally deep form of talmud torah, which, to the best of my knowledge, has no restrictions on responses to dvarim she-bikdushah?

My chavrusa wanted to suggest that learning M"M involves a more direct connection to HaShem than regular learning does, akin to the connection that one creates during the amidah. Tzarich iyyun.