Where you sitting in the Cockpit with the crew at the time ? If not, how do you know if there was an obstruction or not ? The other aircraft might have been off the runway already (or a chopper was crossing nearby giving them a TCAS warning) before you could see anything but by then the pilot already made the decision to go-around (pilots actually make this decision and commiting to land quite a number of seconds before reaching the thresold) and even then it is not the ATC to blame - sometimes it takes longer to vacate than it was anticipated or the pilots did not accept a reduced seperation landing.
Whatever the pilot tells the pax, in this case does not make a difference in their lives, nor does he have to justify the go-around, so if he gave a false reason, I agree that he might as well tell the true reason if anything at all. Why donīt you find out who it was and ask him ? lol