During tl-setup, we look for sendmail in /usr/sbin, and abort the server installation if we can't find it. On Arch Linux, neither postfix nor sendmail packages I've found places sendmail in /usr/sbin - they both seem to install binaries to /usr/bin/.
LSB says that sendmail should be in /usr/sbin. Arch makes no apparent claim to be LSB compliant.
We do require some level of LSB compliance but "Libraries and commands from LSB 4.1" is rather vague. If we want just the libraries and commands, you could argue that it should not matter where they're placed as long as they're in PATH/the dynamic linker can find them. If we want to libraries and commands where LSB says they should be, perhaps we need a more clearer definition of requirements?