FWIW $< is < from redirecting input... a mnemonic for inputs.

$@ is target, because @ looks sort of like a bullseye.

significant tabs are gross, yes, but this is a one-time edit to your vimrc then you can forget about it forever.

it’s also installed everywhere and has minimal dependencies. it could be a lot worse. (see also: m4, autoconf, sendmail.cf)