I don't think so, but we're probably just debating semantics here. In my mind, the real backlog is the orders that haven't been fulfilled LESS the ones that get cancelled along the way.
I'll try to explain it another way.
Let's say Elon says that as of today, there are 600,000 deposits for Model 3 vehicles (ignore the small number already delivered). I put my order in today as number 600,001. If the production rate is 1000 per week and never gets bigger (just to simplify the point), I will not have to wait 600 weeks to get my car. There will be quite a few people in that list of 600,000 ahead of me who change their mind. I just made a guess that it's 50%. pick your own breakage number.
I don't disagree with you that the backlog will keep getting bigger! The car will be very popular. Just trying to point out that forecasts of delivery dates often aren't including an estimate of the people who change their mind and never take delivery. That will speed up delivery by some amount.