Nothing is impossible, especially if you consider we do not know exactly what is involved, how much data and art work has already been completed... remember, PD was likely working on GT5 even before GT4 hit the store shelves, and of course much of the modeling work from GT4 can surely be adapted and enhanced for GT5 with not nearly as much effort.
So first of all, I'd suspect that 6 month quote referred to just new cars to the series, which granted, are likely to be many.
Second of all, all he mentions is 6 months. That could just as well, and very likely does mean a team of artists are working on many cars at once, and that it is a six month process, no matter how many are being worked on at the same time. I seriously doubt they are working on one car at a time.
Thirdly, it is a very common procedure to outsource work when working on such a large project. There is a huge industry of graphic designers and programmers that specialize in contracting work projects.
So no, it is definitely not impossible.
Personally though, there are a lot of complications that could arise with having to rush to get that many cars out in time for a release, and besides... how would they ever make up for that kind of massive production costs?
I suspect we will see GT5 release with possibly even fewer cars than GT4, and then as new cars are developed and released, players can decide for themselves which ones they want, and for their to be some cost associated with that.
Obviously it is all far too soon to be trying to accurately guess what will happen at this point... but if their goal is 900 cars, I can't wait to see what new models make it into GT5. 👍