Why isn't 18 months enough ?
Being sick is not a binary. We've all had days wher we feel like shit but figure we don't feel quite ill enough to stay at home, and so end up taking a couple of painkillers and favourite source of caffeine and end up working.
Presumably the shorter the work day, the more likely you would be to consider it ok to push through it if you feel ill, especially if you have vested interest in demonstrating that a new arrangement like a 6 hour day "works".
I'm not saying they did, but there are many ways they could plausibly sustain something like that for quite a while. The question is whether it is sustainable on a long term basis, or if it would change if/when a 6 hour day is no longer an experiment.
If the net effect is that you still show up to work anyway, does it really matter?
To be clear: I'm not stating a belief in terms of whether or not people do this or not, but it is a tricky confounding factor to exclude, because everyone involved in an experiment like that will have vested interests in the outcome that might significantly affect how they act.
It's not like you can double-blind a study like this. Ultimately it will likely take long-term studies from places that actually implement it on an ongoing basis over much longer time frames to get a true picture of the effects.
That said, as I said, I'm all for working fewer hours. I just don't think figuring out what the cost/benefit tradeoff is will be remotely this simple.
Everyone here called in sick at least once when they weren't actually sick.