The original post and the link and quote from AGEuk comprise what is known as a "counter-factional conditional" in which a possible situation is debated as if it were already actual.
People often act **as if** it were true and some pretty awkward arguments follow.

I don't see any suggestion that 'incarcerating' the elderly is an actual policy in either the US or the UK. There is, of course, a natural concern that because advanced age often correlates with susceptibility to a virus such as COVID, special care should be taken to not only protect the elderly, but also to be aware that IF they mingle freely in society, they might increase the volume of infections.
   A group whose stated objective is the welfare OF the elderly can easily slip into scare tactics in order to preempt something that might happen.

Short version... we need to be careful what we presume....