Saving space by thinning universal intel/arm binaries
That's just how other tools do it, like CleanMyMac does with Universal apps (not sure if it's been updated for Universal 2 yet). There doesn't appear to be a better solution than manually removing the other architecture from each app, which can be done with some tools (see Is there any free solution to remove unneeded architectures from Universal binaries?) and these can be made to loop over a given folder of applications.
There are more steps required than just disabling SIP to do this with Big Sur's own applications now that Big Sur boots from a sealed APFS snapshot. Can I mount the root (system) filesystem as writable in Big Sur? goes into the steps required to make a modification to the bootable snapshot, but the modification you make to remove the other architecture is the same.