When you spread into an array, you call the Symbol.iterator method on the object. && evaluates to the first falsey value (or the last truthy value, if all are truthy), so
let arr2 = ['value2', ...(condition && arr)];
results in
let arr2 = ['value2', ...(false)];
But false does not have a Symbol.iterator method.
You could use the conditional operator instead, and spread an empty array if the condition is false:
let condition = false;
let arr1 = ['value1'];
let arr2 = ['value2', ...(condition ? arr1 : [])];
console.log(arr2);(This works because the empty array does have the Symbol.iterator method)
Object spread is completely different: it copies own enumerable properties from a provided object onto a new object. false does not have any own enumerable properties, so nothing gets copied.