And I see that -Wno-strict-aliasing is actually present in the GCC options! So why on earth are we still getting that warning?

The "-Wall" enables the strict-aliasing warning.
I think the "-Wno-strict-aliasing" would then disable that warning, but then there's a second "-Wall" switch which probably enables it again.

You can edit the WriteMakefile() section of the module's Makefile.PL such that the flags are modified to give the behaviour you're after. (See the ExtUtils::MakeMaker documentation).
I'm reluctant to be more specific because I'm not sure which flags are which. Output of perl -V should identify that for you.
If both occurrences of "-Wall" are part of the same flagset, then I would remove the second occurrence. But if the second "-Wall" is in a different flagset (which is probably the case), then I would insert "-Wno-strict-aliasing" immediately after it.

Cheers,
Rob