There are a few factors at play here. Some of which have already been covered:
1. Their math is wrong. increasing from 600 to 1000 is a 66.6% increase. Not 60%. Sorry, that just bugs me.
2. They are not covering the psychological effects that lead up to a natural adrenaline increase. Those can be extremely powerful.
3. Being loose with regard to power is referring to the antagonistic muscles. Antagonists are the muscles that work in opposition to the intended movement. Any tension in those muscles will slow movement and will reduce power. Think about extending your arm. Your triceps contract causing your arm to extend. Any tension in your bicep would work in opposition to that extension. The bicep must me fully relaxed if you're going to get maximal effort from the triceps. Try driving your car with the brake off and then with the brake on. Same effect.
What we don't know:
Obviously the adrenaline increase causes increased effort in the contracting muscles. Does it also increase tension in the antagonist? Probably so. Apparently the increase in the contraction is disproportionately higher than the tension increase in the antagonist and therefore is capable of overcoming the tension in the antagonist. It's doubtful that increased adrenaline causes improved relaxation of the antagonist. That's just illogical. But, we really don't know.
What we also don't know:
Did the power increase result from increased acceleration (speed), increased muscular tension, or increased body commitment (mass) involved at impact?
Important to note:
Obviously a major adrenaline rush results in the ability to deliver more power. BUT, AND THIS IS A BIG BUT, that adrenaline rush has very debilitating after effects. For a fighter in the ring that can spell big trouble in a long fight. If you utilize a maximal adrenaline rush early on you better finish the job or you may pay the price later.
In summary:
As others have said above, full muscular tension is required at (actually just before) impact and through impact to transfer power into a target. We also know that full relaxation of the antagonists is necessary for maximal effort, movement speed, and acceleration. A motivational adrenaline increase will increase power transfer into a target.
Conclusion:
My conclusion is that one should use the motivational adrenaline increase judiciously and strategically during competition. It works, just like most fighters already intuitively knew. It would be useful to train oneself to utilize that motivational adrenaline increase along with training that results in subconscious/automatic relaxation of the antagonists. The result could possibly result in tactical and practical utilization of the adrenaline increase.