Truncate a String: solution proposed better than Advanced FCC Solution?

Tell us what’s happening:
This is the FCC Advanced:

function truncateString(str, num) {
  if (str.length <= num) {
    return str;
  } else {
    return str.slice(0, num > 3 ? num - 3 : num) + '...';

I just wanna hear your thoughts about the Title question?
The proposed solution:
1)It should be a bit faster
2) use two methods instead of nested conditionals
3) is more readable

Your code so far

function truncateString(str, num) {
  let x;
  num < str.length ? x = str.slice(0, num).concat("...") : x = str.slice(0, num)
  return x

truncateString("A-", 1) // --> A...

Your browser information:

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Link to the challenge:

the solution in the guide is just wrong, it is about an old version of the challenge

your algorithm does a different thing, you can’t really compare them