No, I think that statement was made by the rebuffed (perhaps even "offended") party without citation, quotation, or further explanation. Skeptical, I take it with a grain of salt and would like to hear from the horse's mouth before I start judging people's motives.
I didn't see any statement on their website about the issue or their policies on not accepting donations. I suggest you email CCF if you want to hear directly from them. There is plenty of evidence here to make a judgment on however.
Even taking Gencon's statement with a grain of salt, the other possibilities you posit as their motives should be evaluated for their plausibility, not just their possibility in evaluating the situation.
It is possible somebody held a gun to the statement writer's head ordering them to write the statement for an unknown purpose and CCF never actually declined to accept the donations. Its not a logical impossibility and would change our views of the situation if we found out it were so but it is not worth withholding judging the situation on its face over such implausible possibilities.
Lack of 100% knowledge is not a good reason to withhold judging a situation. It is very hard to get 100% knowledge and you need to judge situations all the time.
Again, we base this on what evidence, exactly? Hearsay? A popular meme of D&D being persecuted by Christians?
The evidence is hearsay, Gencon's public statement of what happened.
I take it we both accept Gencon's statement that the charity took the action of declining to accept the donations, you are only skeptical about the stated motive of why.
Given what we know here, that Gencon made the statement saying CCF declined the donations because it was derived from sales of D&D, we must exercise our judgments about what reasonable conclusions can be made.
I am not aware of any reason CCF would wish to boycott donations connected to Hasbro. I think it unlikely to be over individual auction items. I see no obvious reason for Gencon to be false in making the statement. Given the statement and these alternatives, the reason Gencon gave in its statement seems plausible.
Is there anything you are aware of going on at Hasbro as a corporate entity that would be a plausible motive for CCF to decline donations connected to Hasbro?