What is so hard to believe about the 10 day thing?
I don't know how they run, but seems like it could work. If you have written done, then most of the ground and background knowledge "should" be there already.
So, basically, 10 days of flying, with a little pre and post flight each day.
Full time, 10 days.
Lets see, 4-5hrs a day (fly 2 times a day, 2-3hrs per flight isn't unrealistic) some pre and post, and that is a good 8 hr day. That adds to 40-50hours pretty quick.
Just got my rating done today. Over 3 months, but only 50hrs of actual instrument training time, then a last check day yesterday with the instructor.
I think it would be very easy to go faster if a very detailed plan was set up, and that is what these places specialize in. Then it all becomes a matter of how much a student can retain.