It's not "significant whitespace" in the sense that it is usually used.
Few complaining about significant whitespace think:
foo bar = 3
Should assign 3 to a variable named "foo bar" because otherwise it's significant whitespace. The lexer already splits tokens on unquoted whitespace for most languages, so requiring a comma in lists solves ambiguity.
The only real argument against it would be that this looks confusing (though it's still unambiguous!)
[ 1 2 + 3 4]
so one would either have to live with that or parenthesize multi-token expressions in a list. Note that due to Nix's weird function syntax, parenthesizing function-calls in lists is already required there, but c-like languages don't have this problem.
People who hate on “significant whitespace” tend to have a fairly arbitrary set of conditions for which whitespace being significant, and in what way, bothers them.
