a) If you have monthly payroll that is higher than 4M a month (2.5x4m=10M), than you can cut payroll and not have your total loan amount impacted (although it might impact the amount of loan forgiveness)
b) If you hired a ton of people over the last year, your average monthly payroll costs is going to be lower than your current payroll costs. And seeing as how the comparison period for forgiveness is the first half of 2019, this gives companies a free pass to cut employment to those levels without punishment.
So yeah, not all companies would benefit from layoffs (particularly smaller companies that have no little or no employment growth in the past year), but it seems like at least some could. The caveat here is I am not very familiar with the bill, so I may be missing some clauses that protect against the above scenario.
EDIT: Also, its not clear to me how the 25% salary cut exemption and the max $100k clause play into this. It seems like the payroll ratios that are used to adjust the loan forgiveness amount don't get adjusted. So in the above loopholes, instead of laying off employees, you could cut salaries without penalty. Alternatively, you could cut salaries by 25% but add more employees to the payroll to get the same monthly total in order to qualify for the full forgiveness.