It's a zsh thing.

14.7.3 ‘=’ expansion

If a word begins with an unquoted ‘=’ and the EQUALS option is set, the remainder of the word is taken as the name of a command. If a command exists by that name, the word is replaced by the full pathname of the command.

From: https://zsh.sourceforge.io/Doc/Release/Expansion.html#g_t_00...

This is a zsh thing, it's shorthand for:

cat "$(which egrep)" "$(which fgrep)"

In ZSH `=executable` expands to the full path of `executable`.