It wasn't necessarily the manufacturers advice.
Yes it is the manufacturers advice. The Dash 8 QRH says that if the gear indicates unsafe, you should check the alternate indicating system, if that indicates down and locked then the gear is down and locked*. Obviously that advice is now known to be wrong on this occasion, but the pilots didn't have any reason to suspect that at the time. They did exactly what they should have done and were let down by both the aeroplane and the TAIC.
*From memory, I don't have it to hand and
haven't flown the Dash for 18 months.