> Where does it supports the claim that suspect-customers were/will-be illegally detained?

Not sure what your understanding is, but imo if a person is physically blocked from leaving a store, then they are being detained. It doesn't matter if an employee is doing it or if it's a gate with a receipt scanner

They can and have had employees standing at exits to check receipts in the past, but people are under no obligation to present a receipt in order to be allowed to leave so most people just walk right by

Physically placing a barrier in the way to prevent that is detainment. Replacing the employee with a scanning machine doesn't somehow change that.