As per docs:

The matchAll() method returns an iterator of all results matching a string against a regular expression, including capturing groups. - MDN

So if there is a match or not, It will return an iterator object.

In your case, there will not be a match and will return an iterator and it is considered as a truthy value.

You can get all result and collect into an array and then check the length of that array to check for the match as:

const matches = [...test.matchAll(regex)];

If there is no match then matches will be empty array

const regex = /\+\+\+\+/gm;

let test = `+++`;

const matches = [...test.matchAll(regex)];
console.log(matches);
if (matches.length) {
  console.log(true);
} else {
  console.log(false);
}

If there is a match then matches will be an array of matches

const regex = /\+\+\+\+/gm;

let test = `++++`;

const matches = [...test.matchAll(regex)];
console.log(matches);
if (matches.length) {
  console.log(true);
} else {
  console.log(false);
}

matchAll returns an iterator, which, when coerced to a boolean, is true.

Perhaps you meant match?

const regex = /\+\+\+\+/gm;

let test = `+++`

if (test.match(regex)) {
  console.log(true)
} else {
  console.log(false)
}

Although for this purpose, RegExp.test() is better:

const regex = /\+\+\+\+/gm;

let test = `+++`

if (regex.test(test)) {
  console.log(true)
} else {
  console.log(false)
}