I'm not a copper, but I do have a fair bit of training in security and dealing with armed assailants (healthy homicidal ones, not 93 and stuck in a chair). We are trained, if no risk to life, just run away. Nothing to be gained from tacking an armed person if that armed person can't hurt anyone.
Reality is, you're dealing with a relatively immobile individual with a cold weapon. Hence there is no risk to life if that person is contained (which they were). So, there is no possible valid argument for use of force. I suspect in reality, the officers didn't want to spend the next 4 hours waiting for him to put the knife down and wanted to put an end to the situation quickly. Be that pressure of other calls, end of shift coming up, bad day, whatever.
In truth, patience and time would have resolved it with no need for violence. He was 93 for pete's sake, he'd have nodded off eventually!