I counted about 4 songs with double-bass drumming, which, to me is one of the few objective factors that I think is sufficient (but not necessary) for something to be "heavy metal". I'm not much of a "hard rock" aficionado (well, maybe I am and just don't know it!), but do you have other records that make liberal use of double-bass drumming that you'd call "hard rock"?
I also think that the number of songs with double-bass drumming on 'Keeper II' is not a whole lot higher, and that the sonic gulf between 'Walls of Jericho' and 'Keeper I+II' is far wider than the one between 'Keeper I+II' and PBGA.
That's why I said that 'Chameleon' marked the end of the forward-looking half of Helloween's career. If we count 'Keeper I' and 'Keeper II' as one "album", then amongst their first 4 albums ('Helloween/Walls of Jericho', 'Keeper I+II', 'Pink Bubbles Go Ape', and 'Chameleon'), I would say that the 'Keeper I+II'->PBGA transition is actually the *least* jarring of the three transitions. From that perspective, it's hard to justify PBGA as being "not Helloween", since then only one of their first four albums could be called the "true Helloween", and the other three would have to be "not Helloween". Judging by their start, "always changing, moving forward" would be the primary definition of "true Helloween", which then also prevents their later, albums from being "true Helloween", since they're largely backward-looking.
Though, I completely understand that if you're coming in from a later perspective, the 2nd half of their career could create a much more static definition of "Helloween" in your mind, and against that background, PBGA stands out more.
Neil