Ahh i think these are old instructions, maybe copied from an old blog post or something. You don’t need the arch command anymore (provided you haven’t done something unexpected like run your terminal emulator of choice under rosetta - universal binaries make it easy to make that mistake!).

This form of instruction was popular the first few weeks after m1 launched but homebrew got native support fairly quickly. Initially i had an arm install, /opt/homebrew and an Intel “ibrew” alias for /usr/local/homebrew but even the stubborn binaries now have m1 builds. I think the last hold out for me was the D compiler - not that i write any D, but i have a tool i find handy written in D.