alt Hacker Newsmarket economics are like newtonian mechanics. It's all so wonderful and logical and even elegant, until the dimensions expand a few orders of magnitude, and then all the rules break. Having worked on a trading floor for 20 years I know how this works. Swamping a market with huge trades is definitely considered manipulation by essentially all authorities, and indeed is a form of monopoly power, which even economic theorists will agree is undesirable. Jane Street just got a mega fine for exactly this in India, btw.
I can't get over the confident dismissal of science by hand-waving about imperfect modelling. But what I said has nothing to do with that, and is more true to the real world than an idealized perfect competition model. Pathologizing normal trading behavior like this is more the result laymen and authorities misinterpreting bad economic modelling. So I recommend you take some of your own medicine and look at the mirror. Maybe a trade affecting the market isn't so suspicious as you make it out to be, because the perfect competition model you're using to make accusations of monopoly simply doesn't make sense. Again, if there is something wrong with affecting the market, then you or I are just as liable for our consciously self-interested behavior of choosing higher-quality, lower-priced products.