According to the kernel's documentation:

auto,nosmt:
Mitigate all CPU vulnerabilities, disabling SMT if needed. This is for users who always want to be fully mitigated, even if it means losing SMT. Equivalent to:

• l1tf=flush,nosmt [X86]
• mds=full,nosmt [X86]
• tsx_async_abort=full,nosmt [X86]
• mmio_stale_data=full,nosmt [X86]
• retbleed=auto,nosmt [X86]

Why are these other args being explicitly set in /etc/default/grub.d if mitigations=auto,nosmt is already being set?

There is a limit on number of characters that can be in the kernel args as well, and the kernel args we set will just get longer and longer over time. I don't think it is a good idea to waste the precious characters on these redundant args. Either we use mitigations=auto,nosmt (which should be the default anyways), or explicitly spell out which args to set so the user can easily customize them. There is really no reason to have both.