Funny how cycling is often regarded as a sport easy on the knees. Do they mean leisurely rolling along? Hard riding can be taxing on the knees. Seems ample time to warm up is one of the important rules to abide by. I actually had good results with some massage oil applied before heading out.
This is definitely easy for the knees, even when riding hard and a lot.
Soccer, tennis, basket or running are a lot harder for the knees, a lot.
Tendinitis is the most common in cycling but this is nothing if you compare to cartilage, meniscus or ligament issues.
The only bad thing in cycling is the number of leg revolution. You have to start with a healthy knee because if you have a bit of "play" in the knee, cycling will definitely make it worse.
I did ride my bike as a rehabilitation before surgery of my ACL, and I had a bunch of play in the knee, and I caught a tendinitis in 3 rides.