>> The maths behind this eludes me.

> Dunno, 1st post in this thread was made in 2014,
> it is now 2023, seems simple enough for me.....

Ah, you changed the reference. This explains it. My posting you replied to and all figures therein refer solely to the accomplishment(s) of the fundraising campaign(s), which started in June 2017, not about when the respective development work will be accomplished. Even if my estimation for the collection of the additional 16000 EUR of mid to late 2024 will come true, this won't say anything about the completion of the actual work, which might take years longer or even forever.
Functional prototype within 10 years from project start (2014) is still in reach in my opinion, but a sellable product certainly isn't. It surely doesn't help that SLIMBOOK has EOL'd the selected Eclipse chassis.