Right.... if they have 25, the hard part is done. Now, it's just set up the machine and press go.
Well if the initial production run were done on a MOSIS process (partial wafer with other users), they would need to tape out again for a full wafer, unless they wanted to spin the same wafer over again. The tape out is what costs the most, but having a proven-working design certainly helps things along when you want to graduate up to full wafers.
Wafer sharing is not the only method giving such low-quantity outputs.
It also may be a sASIC with some layers configured by ion-beam etching. Expensive, but literally single chip production is possible.