For what it's worth I travel often with a bike. I have taken a bike as baggage on a plane to/from almost every continent. I'm not a big fan of TSA in general but I have found that, when I am polite and ask nicely, they will allow me to open my bike box and assist them in their inspection, then allow me to repack everything (if necessary) myself. I generally arrive at the airport an extra 30-45 minutes earlier than is necessary to allow this. This ensures there is no rush and the agents generally appreciate my willingness to wait a few minutes. (They deal with irate people in a hurry all day long.) I've never had a TSA agent tell me "no". On several occasions they told me it wasn't necessary as they were satisfied with the x-ray alone.
IMPORTANT - Don't put anything in the bike box that will be suspicious (CO2 cannisters especially, flammable fluids like chain lube or degreaser, etc.). I have had no problems with my bike, wheels, shoes, helmet, pump, and some essential tools all in the box.
Within the United States, that may depend on the airport you use, staffing and equipment availability at the time, etc. You may not be able to be present for any inspection, which may occur out of your sight. When my bike case was inspected in my presence, and I was being very nice and friendly, I was able to provide verbal guidance to TSA employees, but not allowed to touch anything during the bike case opening, inspection, and repacking. On other (most) occasions, I have provided my bike box to the airline agent, and never had the opportunity to see it again until it arrived at the other airport.
Your warning is good advice, as anything you can do which limits the probability that TSA will open your bike case is a good thing. So, it's best if the TSA can be satisified by the scan and not feel the need to open the case.
Fortunately (so to speak) for me, every time my bike has been damaged in transit (always due to improper repacking by TSA !!!!!!!), it's been on the return leg of my trip, so my cycling vacation wasn't affected.