You are right, currently because of the code
it is not possible to assemble and link pure assembly programs under Windows (the only OS with case-insensitive filesystem) because it gets remapped to the C compiler.
To be fair, pretty much all (GNU) C compilers can handle .s assembly files too and that may have been a correct fix there, but not here with SDCC. We just get
Building in release mode
sdcc -DF_CPU=11059200 -DHEAP_SIZE=64 -DPLATFORMIO=50200 -Iinclude -Isrc -c -o .pio\build\stc15f204ea\src\blinky.rel src\blinky.s
at 1: error 119: don't know what to do with file 'src\blinky.s'. file extension unsupported
SDCC : mcs51/z80/z180/r2k/r3ka/gbz80/tlcs90/ez80_z80/ds390/pic16/pic14/TININative/ds400/hc08/s08/stm8 3.8.4 #10766 (MINGW64)
I will file a bug in the core.