Yes Tom, at first I thought nah, it can't be, there are thousands of references to Sloterdijk on-line and in virtually all cases it is with an "ij" which I believe is the proper Dutch spelling. In my home language Afrikaans, we use a "y" and maybe it was a mistake made when the ship was painted in Cape Town. It wasn't and you are 100% right!
Traditionally HAL Passenger ship names end with "dam" and cargo ships used the "dijk" suffix early on but it appears that sometime after the First World War it was changed to "dyk".
Here is a photo of M.S. SOMMELSDYK in 1939 (under Danish flag), her name on the hull clearly spelt with a "Y":
Many more photos of HAL -DYK and -DAM ships and the story of Captain "Kees" Haagmans here:
Incidentally, Kees Haagman mentions being on board M.S. SLOTERDYK in New York during early January 1941, at the time when the Buffalos and Falcons were probably loaded.