It says premium only on the gas cap.I can see how you'd interpret it this way. But if you pay attention to this bit: "At minimum, the gasoline you use should meet the specifications of ASTM D4814 in the U.S.A..", one could reasonably conclude that there is no minimum octane requirement as that standard has no octane rating associated with it yet is clearly identified as a minimum standard. You've also demonstrated Toyota elected to print "Unleaded Gasoline Only", twice, when they could have written "Premium Unleaded Gasoline only."
The manual for German sedan I once had never introduced ambiguity like this. Maybe it's a Japanese to English translation issue?
You may be right, it may be the case that premium gas is required to avoid engine damage, but it's not laughable to question that given the info we have so far. Hopefully Toyota will give a clear answer.