I have always understood acqui-hire as "Big company sees a team of talented devs with good cost structure (aka: manageable salaries) so they pay a one time flat fee to get that team working on their problems"
At no point have I ever expected developers salaries to change during an aqui-hire. They might get a boost after the aqui-hire if they have trouble retaining the devs for whatever reason. But usually the whole point of an aqui hire is the relative low salaries of the employees compared to their performance.