I do not doubt that freezing them would kill the bugs. I would be worried that unless it is very carefully managed it might damage the books though. In particular i would worry that moisture from the air would freeze on the books and as they are thawed they would get water damaged. Or that moisture trapped inside the bindings would form ice crystals and physically damage the books as they form.
None of these are concern with the hypoxic treatment they choose. Plus the nitrogen atmosphere treatment is so much simpler on the practical level. Instead of bringing in freezers and powering them for the whole duration of the treatment all you need is some crates, plastic bags and nitrogen bottles. Makes it much easier to bring the treatment where the books are, thus you avoid all kind of complications with transporting the books.
Well, it's an idea. Perhaps de-humidifying the books first...
The hypoxic approach needs to last at least until eggs hatch, otherwise you're back to square one. And I'm not so sure if a plastic bag can hold tight for long without leaking (nitrogen out, air in).
None of these are concern with the hypoxic treatment they choose. Plus the nitrogen atmosphere treatment is so much simpler on the practical level. Instead of bringing in freezers and powering them for the whole duration of the treatment all you need is some crates, plastic bags and nitrogen bottles. Makes it much easier to bring the treatment where the books are, thus you avoid all kind of complications with transporting the books.