An empty array is truthy (usually anyway) so even when getElementsByTagName(i) finds nothing and returns an empty array, the "then" block is still the one that gets executed.

console.log('is [] true?');
if ([]) {
  console.log('yes');
} else {
  console.log('no');
}

console.log('---');

console.log('is [99] true?');
if ([99]) {
  console.log('yes');
} else {
  console.log('no');
}

If you're wondering why I said "usually", see https://xavierchow.github.io/2015/12/15/js-coercion/ but don't worry if you can't follow it. It was news to me as well.

According to the docs, document.getElementsByTagName(i) returns a live HTMLCollection. No where in the docs does it say it returns anything other than that. So it is safe to say that document.getElementsByTagName(i) will always be truthy (i.e. not null or false). You can instead check the length member. It will be greater than 0 if at least one of the element exists.

function headingsEx() {
    let h = ["h1", "h2", "h3", "h4", "h5", "h6"];
    for (let i of h) {
        if (document.getElementsByTagName(i).length) {
            let foundHs = document.querySelectorAll(i)
            for (let c of foundHs) {
                console.log({ [i]: c.textContent })
            };
        } else {
            console.log([i] + " doesn't exist");
        }
    }
}

headingsEx();

Since you asked about alternative approaches:

This example grabs all the heading elements using querySelectorAll, and then uses map to create a new array of node names. You can then iterate over the headings array and check to see if each element is included in the nodeNames array, and then log the result accordingly.

const arr = ['h1', 'h2', 'h3', 'h4', 'h5', 'h6'];
const headings = document.querySelectorAll(arr.join(','));

const nodeNames = [...headings].map(heading => {
  return heading.nodeName.toLowerCase();
});

arr.forEach(el => {
  if (nodeNames.includes(el)) {
    console.log(`${el}: exists.`)
  } else {
    console.log(`${el}: does not exist.`)
  }
});

<h1>Hi</h1>
<h3>Hi</h3>
<h4>Hi</h4>
<h6>Hi</h6>