This is very true, and cannot be overstated. Often, the criticisms of one theory are actually much better direceted at a predecessor theory. Students in a history of economics course, or a regular economics course, may learn of the "labour theory of value", but they may learn either Smith's, Ricardo's, or Marx's, and assume that the theories are the same, or that the same criticisms apply to all of them.
There are good and serious criticisms of the "labour theory of value" from those unsympathetic (and even those sympathetic) to its apparent normative conclusions. It's a shame that many people seem to confuse one thinker for another, especially when each boasted of their improvements (Marx, for example, writes "I was the first to point out and examine this [...]" when writing on the "LTV" and discussed the law of the tendency for the rate of profit to fall again in his original understanding, despite being extremely well read on - and critical of - his predecessors).
The theory is that surplus value (which can become profit, or be reinvested, etc) exists only through paying the worker less (on aggregate) than the aggregate amount that the worker provides through their labour.
The mainstream economics answer to this is to say that the labour theory of value is bunk and that prices are defined purely by market forces. But this kind of misses the bigger picture of what Marx was getting at rather by getting lost in the weeds about price definitions and really is a critique of Ricardo more than Marx.
Despite using the term "exploitation" which has moral overtones in English, Marx really isn't casting a moral judgement here. It's a technical description. But he does believe that this phenomenon leads to structural inequality and injustice and that the only resolution is some other kind of property ownership or method of production. Though he was famously vague on what that alternative would be, and people far less intelligent than him were left to fill in the blanks with some rather awful alternatives.