It’s an internal command that’s part of L:shell-seg. If you’re in a shell after it’s been made resident as Thomas described, then ALIAS will be available.
I think it’s the same way CD, If, Else, etc. are internal in Kickstart 2+. Those commands aren’t stored directly in the ROM; they’re part of shell-seg, which itself is part of the ROM. The difference in 1.3 is that shell-seg is loaded into memory from disk instead of being automatically/instantly available from ROM.