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);
}

Continue calling re.exec(s) in a loop to obtain all the matches:

var re = /\s*([^[:]+):\"([^"]+)"/g;
var s = '[description:"aoeu" uuid:"123sth"]';
var m;

do {
    m = re.exec(s);
    if (m) {
        console.log(m[1], m[2]);
    }
} while (m);

Try it with this JSFiddle: https://jsfiddle.net/7yS2V/