TBF I think it is a very real plan, albeit we probably haven't seen all of it yet.
Putting aside the moral questions for a moment, this has zero chance of working as it is - there isn't the capacity to house these people securely, the staff to process them, the patrols and personnel to pick them up at sea or on landing or further inland. There is no indication in this bill (or in the associated spin) that there will be any of those things anytime soon. The only way it would ever work is by massively reducing the numbers coming by this route, which means establishing alternate means of seeking migration or asylum (ie: allowing applications from a third country with a good chance of success). Doing that would also remove nearly all of the legal force against these measures too, but if they announced that first (or simultaneously) they wouldn't get the culture war hit that they are seeking.
At a guess, I would expect some apparently sensible loyalist backbench Tory MP will propose some amendment or other allowing asylum applications from outside the country, in the name of fairness, British justice etc etc. The government will reluctantly accept this common-sense measure and will proceed to create an underfunded system that takes ages to make a decision.