Quote from Hicks, PD09 (emphasis added)
9. If all pleasure had been capable of accumulation,--if this had gone on not only by recurrence in time, but all over the frame or, at any rate, over the principal parts of man's nature, there would never have been any difference between one pleasure and another, as in fact there is.
It seems to me (check my Greek, please!) that the specific construction of PD09 falls under the unreal present as defined in that Wikipedia article and elsewhere:
Present unreal conditions
Unreal (counterfactual) conditions referring to present time are made with εἰ (ei) followed by the imperfect indicative in the protasis, and the imperfect indicative combined with the particle ἄν (án) in the apodosis
 [ιχ.] Εἰ κατεπυκνοῦτο πᾶσα ἡδονή, καὶ χρόνῳ καὶ περὶ ὅλον τὸ ἄθροισμα ὑπῆρχεν ἢ τὰ κυριώτατα μέρη τῆς φύσεως, οὐκ ἄν ποτε διέφερον ἀλλήλων αἱ ἡδοναί.
"The unreal conditionals either describe a situation which is contrary to fact, or which is unlikely to happen." (Source)
So, my contention is that, while PD09 is worded rather idiosyncratically, Epicurus is basically saying that pleasures do differ from each other and they can't actually be condensed. Which makes sense to me since he says elsewhere in PD18 that: 18. "Pleasure in the flesh admits no increase when once the pain of want has been removed ; after that it only admits of variation..." How could there be variation if all pleasures were the same?