OK, I can kinda see that for the stuff at the top. However, most of the icons at the top correspond to a labeled item in the left sidebar, and each has a tooltip as well. I agree this isn't the best but I do not readily imagine a better way, either.

As for the stuff on the message level, left-pointing arrow has meant "reply" since the very first graphical MUA, the star is universal design language across Google properties including Chrome, and the three vertical dots for more actions is very common within and without Google.