I caught a Shiny Kyogre that had the lowest possible IV's. Somebody told me that it's rarer than a perfect shiny Kyogre. Is that true?

It would be equally rare...

Basically when you calculate IVs from a raid, you roll 3 dice and add 9 to each result.

So a 15/15/15 (perfect) requires you to roll 6/6/6.
A 10/10/10 (worst) requires you to roll 1/1/1

Both of these have a likelihood of 1 in 216.

Any stat distribution has the EXACT same likelihood though. A 14/11/12 also has a 1 in 216 chance of occurring. It's just that people won't really differentiate that from a 14/12/11 or 14/13/10 or 14/10/13 or 13/11/13 or etc... and thus they think it's less rare. It isn't though - any specific set of IVs (including order) is equally rare.

So what I'm ultimately saying is that while some people may disillusion themselves into thinking Zero IV Pokemon (or 10/10/10 raid pokemon) are "cool" - they're just proof that you got the worst luck possible. There's nothing else special about them.

0 IV = transfer

So yeah rarer than 100IV shinny