newshound wrote
Rod Speed wrote

They would put their nose under the bit of horseshoe on their side, and
lift it up.

Not possible as he shows at 75 secs
https://youtu.be/Y3yGztiAsOU?t=75

Some might use their lips.

Again, not possible to lift the ring that way for a horse.

American barn doors are typically solid up to about 4 feet, and the bit
above has vertical bars essentially like prison bars, in a box-section
frame. To unlatch, you grip the bar nearest the opening, and slide it
upwards a few inches. They are quite stiff. But once a horse has figured
it out, it uses its nose or lips to do what you would do with your hand.

But that isnt possible with his gate latch. Only a human or monkey can
lift the ring.

He might make that claim

It isnt a claim, he shows it cant be done by livestock.

and it might be true for cattle.

For all livestock in fact.

But the whole of the moving bit is solid, so it could be raised from the
horseshoe by applying a suitable moment

Nope, because the movement has to be applied to
the big pin with the ring at the top of it and only
the ring is accessible with the gate latch locked
and no horse has anything that can lift that ring.

as well as a straight upwards force.

The only thing a straight upwards force can be applied
to is the ring and no horse has anything that it can use
to apply a straight upwards force to that ring.

Horses can untie knots with their teeth,

But there is no way to get their teeth onto
that ring because their jaw wont fit between
the gate post and the gate vertical frame.

I am confident that some could defeat that. And mules even more so.

You have no basis for that confidence.
Its basic physics, neither have anything
to grip the small ring with and thats the
only way the latch can be released, with
an upward force on that small ring which
is in the small space between the gate
post and the vertical gate frame.