document.getElementById('some-form').getElementsByTagName('input').forEach(i => i.classList.add('error'))

That's also doing things "the right way" and not do-all selectors, but that option exists now, too:

    document.querySelectorAll('#some-form > input').forEach(i => i.classList.add('error'))

   const $ = document.querySelectorAll
   const addClass = (className) => (item) => item.classList.add(className)
   $('#some-form > input').forEach(addClass('error'))

Of course now we have additional defensive programming tools like `?.` and `??` operators.

Point is that the DOM API version of the initial jQuery snippet are not equivalent. $() statement does a bit more than that.

This becomes obvious if the requirement are a bit more complex than the example, perhaps we want to chain more expression on at the end, then the DOM API version becomes more convoluted.

    document.querySelectorAll("#some-form input").forEach(e => e.classList.add("error"));

Same thing of course applies to people who throw around the optional chaining operator without thinking to please TS.

Or would apply if one used a single selector here and document.querySelectorAll

The needless chaining here is a typical example of misleading jQuery code though. Looks as if it was there to prevent this issue.

  for (const input of document.querySelectorAll('#some-form input') {
    input.classList.add('error');
  }

In fact, this has an even earlier baseline browser compatibility than

  NodeList.prototype.forEach