People so often overlook the role that execution plays in trading. As time scales shrink the impact of execution increases, and indeed many HFT strategies are not profitable without top tier execution (both in terms of fee schedules and technology). This is also one of the places that most academics fail when analysing a trading strategy. They make typical "assume a frictionless surface" types of assumptions that break down in a real market.

To your point, high-frequency simulation is damn hard and it is highly likely that they failed here and have tainted their results with bad simulations. Most of these papers would be well served to avoid dollars and cents and simply analyse the statistical qualities of their signal to their target over time. That is the first step in this business, anyway. Simulations only happen once the signal has been through the wringer.