Some degree of truth some degree of total BS. See
http://www.arb.ca.gov/msprog/obdprog/obdfaq.htm and search for the word "readiness". Apparently on an OBD II system if codes have been recently reset or repairs made the system takes a bit of driving before it essentially gets to the point where the system is ready to start monitoring and until that happens there is a not ready flag set. The above site says drive for a week or so to clear all the readiness flags.
I presume if you failed because of readiness flags they were telling you to drive a bit before coming back. On the other hand if they did not do an OBD II test at all and just told you to drive that's BS.