Yeah, that is another dark corner. I was not quite precise (I forgot this behaviour existed): $ expansion does allow for parentheses as well, so that arrays work. Meaning $($b) is the same as [set ($b)] which is element $b of the array in variable {} (empty string; that is the name of the variable). The docs [1] say:
> $name(index)
> Name gives the name of an array variable and index gives the name of an element within that array. Name must contain only letters, digits, underscores, and namespace separators, and may be an empty string. [...] Command substitutions, variable substitutions, and backslash substitutions are performed on the characters of index.
So that means:
set b a
set (a) a ;# this sets element a of the variable {} to a
puts $($b) ;# prints that element
info vars ;# all variables, note that {} is in there
puts $($($($b))) ;# Of course this works...