It's true that intrinsic value is not usually well defined (except in simple cases, where a company is going to be liquidated and has clearly defined assets and liabilities that can be precisely measures, such as cash/marketable securities), but that doesn't mean that a thoughtful investor can't determine a reasonable range of likely values that errs on the side of conservatism. For example, by zero-valuing assets that are hard to quantify, such as an undeveloped plot of land. Or if there is an annuity-like cash flow stream from a long-term contract, one can discount those cash flows to the present using a conservative (high) discount rate. In cases where the share price is way below a conservative downside-case valuation, it's not necessary to know with precision what the intrinsic value might be.

Individual investors can define a proxy for intrinsic value. But, overall investors will come up with different numbers. Thus investor A may think a buyback was a net gain, while investor B may think a buyback was a net loss.