I don't think it's remotely shitty. It's not like the crossover would have been the only thing their recipient wanted (if it was I'm sure we would have seen that person's letter pop up for being an asshole who only wanted a crossover fic), so presumably they wish they'd written for one of the prompts that would have made their recipient equally happy and not lead to it getting passed over by all the other readers who only know one of the canons.
Re: Things I learned this Yuletide