As Pat also said, tire screech noise is not confirmation of wheel lock. A tire slipping some percentage will squeal but is within the traction range that ABS is capable of detecting and correcting for.

Your car has three channel ABS. This means that each front wheel is individually regulated hydraulically but the rears are done as a combined pair. The mean speed of the rear is used for decisions on brake regulation and sometimes one wheel may be slipping say 25% but the mean speed is not deviated from normal decel. rate to cause ABS to fully activate.

I'm not sure what the tech was saying but the brakes on that car have a hydraulic layout where one part of master cylinder supplies fluid to rears and the other supplies the fronts (99 is different). The old non-ABS Volvos have the dual triangular split system where one section of M/cyl supplies 1/2 of each front caliper and one rear caliper and the other section of M/cyl does the other 1/2 of fronts and the remaining rear caliper.