Yes, unfortunately this is true. Pilots are children. And since we're all legally obligated to monitor guard at all times when able, these guys know they always have an audience. Things I've also heard on guard include:
• someone barking, which was nice variety and very much appreciated to see more canine representation in my field
• airline captains making full cabin announcements, not realizing they were transmitting, followed by several other pilots asking questions as if they were in the cabin
• airline captain making about half a cabin announcement before stopping cold, pausing for about 3 seconds, and mumbling "...I am so fucking sorry," then presumably making the announcement again to his actual cabin
• two pilots for different airlines speaking Italian to each other (one of these was actually my captain, the other guy was a friend from his last job)
• pilot doing a radio shockjock voice saying "aaalrighty folks you're listening to one-twenty-one-point-five the GUARRRRRRD, bringing you the lastest in aviation disasters, mishaps, and tragedieesss! Up next, four more hours of nonstop cat soundssssss!"