Personally I regard inflection points as points at which the perception of a company's future prospects by the market does or should significantly change, such as moving from loss-making to being profitable. You could use the term is this instance to describe the declaration of a Mineral Resource Estimation I guess, but with explorers and developers in the materials sector I tend to think of such events as simply stepping stones in their journey, and not as significant as an event I would describe as an inflection point.
MRE's can be received and responded to by the market in many different ways. It really all depends on expectations. They could announce a great MRE, but if the market was expecting an even better one, they could be sold down. If it is generally in-line with expectations, they could not move at all. If it surprises on the upside, it could certainly be a catalyst for a positive market re-rating of the company.
At the end of the day, an MRE is still an estimate, based on JORC code guidelines, and investors know that what they end up mining, if they get that far, could be significantly different by the time they get to that stage.