I think I personally know many people that will simply use the extra time to work another job, thus erasing the intended benefit to the generous company.
They work you less so that you will have better morale and energy and loyalty while you are there. But you use the time to collect a 2nd paycheck, and the gets the same tired employee they would have had if they just worked you fully the old way.
I don't know what could practically be done about that.
It also seems like a 6 month trial where the participants are still the exception not the rule, doesn't really predict what would happen if it became the rule. If most companies adopted this as a norm, surely simply the average salery would just inch down over time until it became essentially necessary and the norm to work 2 jobs, since the 4-day job won't actually be enough by itself except in theory, like how minimun wage is supposed to be enough in theory.
There will always be huslters aggressively trying to maximize every minute, and they will always set, or at least perturb, the going rates for things. I think this experiment can only really exist as a special bubble experiment where the ceos just wanted to do it, and all employees were convinced to voluntarily play along and not just fill the gap with a side job. Maybe there can always be some few companies that can do it as their particular distinctive policy just like there already are different companies with different cultures and norms. But across the board? I think the hustlers and hungry scrappers will just set the going rates.