can anyone explain why my switch statement is printing out default?

That's not the way a switch works. The expression "x % 3 ===0" returns true, but your variable in the switch (x) is 30, so the condition is not correct

The switch statement expression which is (x%3===0 && x%5===0) returns true, therefore if we replace the whole statement to the boolean expression returned by the statement, we get

switch (x) {
case true:
(result = "fizzbuzz"); break;
}

and since x equals 30, and not equals to true therefore the condition doesn't match and passes by until default is hit (since x%3===0 and x%5===0 conditions also yield true).