Their secret sauce, at least according to marketing was composite materials and high bypass turbofans, which combined could make a drastically more efficient plane than the Concorde.
And composite materials like on the A350 and 787 have been around, see the A350 and 787.
The plane doesn't exist either, it was a plan.
> And composite materials like on the A350 and 787 have been around, see the A350 and 787
Their point of comparison is the Concorde. If they can make a plane that is significantly cheaper than the Concorde, thanks to the great tech that is now mainstream (composites, turbofans, CAD) but wasn't back then, it can be similar in cost to modern day business class, while still being supersonic. At least that was their marketing pitch, i don't see it taking off without an engine.