It is clear that when they say they don't advise elderly/other at risk people to isolate yet, it isn't that isolating now is not protective or helpful. You would be safer if you isolate from now but they don't think people can do it and they don't want them giving up and going back into society right at the peak of infection.
It isn't 'too soon' to isolate in terms of risk, the risk is out there. They think it's too soon from a point of view of people's behaviour. If they can only isolate for a few weeks, do it at the peak.
I am advising my elderly relatives to isolate as much as they possibly can.
They want to isolate the elderly and at risk when infection peaks to limit the damage. They know that by not isolating either side of the peak (including from now) many elderly and at risk will catch it and die. They will form a high percentage of the loved ones mentioned in the quote.
They are obviously convinced it would backfire to isolate now. If you think your relatives or you if at risk can do it, do it.
If we all do the maximum with can, more than the govt tell us to, to reduce spread, we may help to relieve the pressure a bit. We just need everyone to do it.