The faqs (frequently asked questions) on Sabre/Scimitar include:
11. How does your frost control technology work?
Very well, thank you! (We aren't giving everything away!)
This appears to be a fudge. It is icing that is the biggest reason why Sabre/Scimitar will not in my considered opinion work and they simply dodge the issue by claiming to have a "secret" solution.
I don't believe they will get working heat exchangers and I don't believe they have any way of dealing with the icing problem.
Edit - The above opinion was based on the originally cited -150 deg C precooling and does not apply for 665 deg C precooling.