Thats not quite how things work.
It means that the estimate is now 20 weeks for orders placed now, and depending on the rate of orders going forward that will affect how quickly that gap closes.
Generally I would use a 1.5x estimate for something like that so 20 week lead time now I would estimate in 30 weeks to get the lead time to 10 days. But that is a very generalized estimate process and not one I would estimate based on Purisms past performance with estimates, it’s really just to provide some context for why a 20 week lead time does not mean 20 weeks to cleared backlog while taking on new orders.
I do agree that it means Purism estimates having all of the orders through today cleared out in 20 weeks from today, but the orders over the next 20 weeks aren’t included in that time period they’ll be included in the estimates available at the time of their purchase.