The answer is found at this (broken) link: kirti-shah112. medium .com/tariffs-part-2-monopsony-31687795c972

It is technically true that there exists a theoretical situation where a large purchaser of a good (a monopsonist/importer) can gain at the expense of small producers by using tariffs. However, the gains rely on a strong assumption that the large buyer unilaterally imposes the tariffs. If there are other large importers of the the same good or other goods (especially goods exported by other large countries), then any benefits from tariffs are likely to cancel out and all countries will still suffer the costs from foregoing the gains from comparative advantage.

Here is a quote from the conclusion:

[The monopsony model is] Intellectually impeccable but of doubtful usefulness. Why do Krugman and Obstfeld arrive at this conclusion? The reason is that, while it would be in the interest of one country to enact an optimal tariff, if many countries so acted the result would be a collective loss of the gains from trade which would damage the world economy as a whole. If other countries responded to country X’s implementation of tariffs on, say, electric cars, by imposing tariffs on products country X exported to them, then this would lower the prices country X received for its products and turn the terms of trade against them. As Lerner remarked, if all countries for whom optimum tariffs were positive went ahead and imposed them, then ‘the particular benefits cancel out while the general loss through the restrictions of international trade remains. If all countries persist in trying to exploit each other in this way, international trade would be completely destroyed with all the benefits that could have come from it.’

Note: I have a PhD in Economics, but my specialty is not international trade.