I generally don't believe any claims about x number of weeks from seed to finish. There are to many factors for anyone to make that claim.
I've only grown a few autos as experiments, but they all still grew like normal plants. I just didn't have to change the lights to flower them.
Once the seed germinates it typically takes 1-2 weeks for the seedling to develop a root system and produce it's first real leaves.
Then another few weeks of veg for the plant to grow and genetically mature to the point that flowering gets triggered.
Once the flowers begin to form I totally believe it will take 8 weeks for the flowers to mature properly an put on weight.
That also doesn't even begin to talk about environmental factors or genetics and phenotype!