You are correct about the lemon cranks being the problem not the nitro...........
Those lemon cranks held up good to 15% nitro usage for years without failure. After running a few tanks of 30% nitro, many of them broke.
Now it appears that the manufacturer has gotten smarter. Instead of improving the crankshaft and costing them (manufacturer) additional money, they have now reduced the carb throat size and may have reduced the compression ratio. This isn't even counting the possibility of reduced valve sizes and the possibility of mellower cam profiles.
If they feel that their niche is inexpensive sport engines, it is easier to accomodate the 30% nitro crowd with changes that cost them nothing or less when compared to hardening the crank sufficiently to withstand the higher nitro.
I'm just guessing, but it appears that in view of the recent production changes to their engines, they are headed in this direction. Change costs money. They certainly had a reason for detuning their already anemic engines.