LTV doesn't claim that any labor creates value in any context, but rather that all value is created by labor - i.e. it recognizes the distinction between productive and non-productive labor. From there then follows that if those who performed the productive labor to create the value don't receive the fullness of what they created, it is exploitation.
(Note, by the way, that this should account for all labor involved in production, including management. The common objection to charges of exploitation is that business owners do create value, and it's valid - to the extent that do, in fact, create it, as opposed to contributing resources that were acquired through similar exploitation from some other creator. Thus, a business owner who is not involved in any managerial decisions, but rather delegates it all to a hired manager, derives their entire income from exploitation; but for a manager-owner, the part of the income that they would receive for the same performance as a hired manager cannot be considered exploitation.)
Note that LTV is not necessarily a leftist thing, at least as "left" is usually understood today. For example, Lysander Spooner promoted policies, many of which would be considered radical right-wing libertarianism today - but he based it all on his understanding of LTV and ownership of wealth that one produces.
It's also not contradictory to price discovery via markets. Left-wing libertarianism includes free market variations thereof. In fact, some of them emphasize the free market as the foundational concept for a truly egalitarian society, and criticize capitalism on the basis that it is anti free market, and this is exactly what makes exploitation possible to begin with - see e.g. https://en.wikipedia.org/wiki/Free-market_anarchism (but keep in mind that not all left libertarians who subscribe to such ideas are anarchists).