Yes it is true. US travel agencies report their sales to airlines thru ARC. ARC rules allow end of next business day to void out a ticket without regard to non-refundable/cancellation fee restrictions. Note this only applies to the airline ticket itself. Any ticketing fees the travel agent charges on its behalf are not covered by this.

If you ticket on Friday the travel agent can actually courtesy cancel aka void the ticket out end of day Monday even if the flight is on Sunday and if Monday is a holiday, the travel agent have until end of day Tuesday to void.

This is what the travel agent is allowed to do. Whether they make it available to the passenger and/or make it available without a service charge is a separate topic. In my experience, both Orbitz and Expedia do offer the weekend courtesy cancel.