The truth, as usual, may be somewhere in the middle. The "general guidelines" are meant to apply to everyone, including a 55 year-old, basically sedentary male. They also assume that this is a person who has "normal" contact and follow-up with a physician, which means doctor appointments weekly, at the most.
Backes is not 55, not sedentary, and has access to team trainers who can monitor his elbow and report daily to the team physician. And, unlike most of us, he's willing and able to upend his life and livelihood to concentrate on healing his elbow, while most of us would have to go back to work and might be able to do a little bit of "home physical therapy" by ourselves for an hour the side, at the absolute most.
In fact, Backes' job now is to do literally everything he can to get healed.
That said, there are limitations to what the body can do, athlete or not. It's definitely NOT good news for the Bruins, since Backes is arguably the most important addition the Bruins have made this year, and the second most important addition is out for three weeks at a particularly inopportune time.