It’s not saying that one has to be able to sustain 13 MET over a very long time. Cardiac stress tests are based on different stages of treadmill speed and slope, with a standard test consisting of 7 stages. Each stage is three minutes long, and you start at stage 1. The test ends when either you have chest pain, shortness of breath, EKG abnormalities, are unable to keep up the pace, or finish all 7 stages. Finishing stage 4 is 13 MET (3 minutes effort). Only a very fit person makes it to stage 7. An average middle aged man should make it to stage 3. PADI is saying that a doctor should check if a potential diver is just a bit fitter than the average middle aged man. Basically, can you run at an 8:00 mile pace for 3 minutes without shortness of breath, chest pain, or other discomfort?
Disclaimer: This is not medical advice or a suggestion to go try this right now. If this is in question, consult with your doctor. It’s just an explanation in real life terms of what the 13 MET stress test is similar to in real life, as an 8:00 Mile is 13 MET effort.