Think about it if asked you to overstate Tolkein's achievements if they were great enough, no matter how hard you'd try, you wouldn't be able to over state them.

I agree that is the usage of "Cannot be overstated". I don't understand why you would apply a separate scenario from what was written.

"Cannot be understated" is a phrase used to emphasize the importance or significance of something. I think that applies. (Google - ludwig.guru summarizer)

That sentence, taken as it is written, would imply the exact opposite to me. There may be dialectal differences, of course, but if there are then people are bound to misunderstand this.

If you cannot understate something, then any statement no matter how bad (for example "it is mediocre") is likely to be an overstatement. That's now how you would think of something important.

Double negations are always tricky. "Can not be understated" can more clearly be written as "easy to overstate".

English slang is full of double negations. A sentence such as "I didn't do no harm", which would absolutely mean harm was inflicted, can mean the opposite. So it may be good to avoid them as to not confuse non native speakers.

I thi k maybe it can be used in both ways, like " I cannot overstate the importance of this" or "you can not understate what I just said" cannot as in "I am not allowing you to understate what I said". Idk, that's my most generous reading of it.