Doctors make decisions about who to treat and how to treat them all day every day - it's what being a doctor is about. It doesn't make sense to give a very elderly person with multiple illnesses chemotherapy for cancer for example - the treatment is very unlikely to work and will probably only cause further suffering. It's not a matter of denying treatment, it's a matter of making an informed decision about what is likely to work.
Guidelines for making treatment decisions are NOT about denying treatment. They are about making the right treatment decisions based on the available evidence.
There is, currently, no reliable treatment for a virus - it is up to your immune system to fight it. That is why the main advice for dealing with covid is to stay at home - for most people, time will do the trick. 'Treatment' in hospital involves doing things to make you comfortable while your body fights the infection and treating any other illnesses you have that could make your chances of survival worse (like diabetes). If it gets to the point where your body can't fight the infection any more, for whatever reason, then there is nothing left to do but to make the person comfortable as they die. It's awful, but, and I can't state this strongly enough, it is not the case that people will be denied a reliable, effective treatment that could cure covid, because such a treatment does not exist. It is not the case that doctors will be letting people they could help die. In many cases, the person will either get better or not, and nothing the doctors do will make a huge difference beyond making the person comfortable.
Ventilation is a very invasive and painful procedure requiring intensive care and doctors will avoid doing it if at all possible. If the person is at the point of needing ventilation, their chances of surviving are already very low. Doctors may decide that ventilation is not the right option at this point. It's very sad, but again, it is not the case that the doctors are denying treatment that could save the person, it's that they're making an informed choice about what is right for that person.
It is absolutely right and true that every effort should be made to keep the numbers of people going into hospital as low as possible by limiting the spread. It's also true that if you get covid, even if you have diabetes, chances are you won't need hospital treatment at all and you'll be absolutely fine. What we're trying to avoid, as a community, is giving covid to people who won't be fine, who will die from it, regardless of what treatment doctors can give them.