Variation in price doesn't prove that the market cap is not (a good estimate of) the company value for highly liquid stocks.

Value is subjective. Stock prices measure peoples perception of the value. Your thesis that it is incorrect can only come from 2 places (I think)

1. Dumb money - the market cant see that XYZ is overvalued or undervalued. My rebutal there is nonetheless XYZ has been valued by a conpletely open continuous auction that people are not restricted to participate in.

2. The parts are less than their sum. This may be somewhat true... total control over a company may be more (or less) valuable than splitting. But I dont think it is order of magnitude. And if it is, it is because the value to you isnt the value to me (the value of RAM to a gamer < value of RAM to OpenAI).