Thanks for this question! I recently re-learned this info, and the way I remembered it (I know it's probably a bad mnemonic but it works for me) is: "COUNT (ALL col) does not COUNT ALL" -meaning it excludes NULLs.
Interesting question. "The ALL will be applied to all values, with the NULL eliminated" may be incorrect. I tried running the same query without COUNT() and found NULL value in the result set. SELECT ALL tc.varcharcol FROM dbo.TheCounts AS tc