I know there's an unspoken rule to not speak directly about whatever shady thing your organization and leadership is doing but for this situation it would be surprising if there isn't some correspondence that would come up in discovery detailing the strategy. It's too much of a coincidence and too big of a decision to hire 20% of your workforce as interns and then a couple months later fire 20% of your employees.
You would need some class action lawsuit I’d think? Need a good number of laid off people to join it and you need solid statistics that would convince a jury more than the corporate lawyers would with whatever HR covered their assess on paper have.
No I'm saying you lay off your senior people. Not because they're old and you don't want to pay retirement but to save on payroll since they make more money than juniors.