There is a difference between:
1. Cars that are similar and just differing slightly in cosmetic/performance due to different model years (e.g. all the various Skylines and Miatas)
2. Cars that are straight up duplicates, identical in every way except for the name due to regional variations (e.g. MX5/Miata, Clio/Lutecia)
3. Multiple version of race cars with the same performance, but different liveries (e.g. NASCAR, base model cars in GT6)
I'm okay with #1, even though I would just prefer the best performing/most modern version.
#2 is a definite no-no, but in terms of development time they cost nothing to PD honestly.
#3 is largely redundant with livery editor, and PD seems to get this by only including one of each Super GT car in GTS.
So yes, the 1000 cars is a myth. The actual number is up for debate however. While I don't care about various model year Skylines, there may be a Skyline nut in Japan who does and wants the exact model he owns IRL for example. At the end of the day, we know PD likes to pad numbers, because it's what sells the game. I would love to see a more varied car list like Forza in GT7, but PD being PD, old habits die hard.
Meanwhile, my Assetto Corsa car folder is currently at 1805 cars (and counting!)