No, of course that is totally a valid reason. In such case Twitter would have misrepresented itself and thus made it impossible to effectively value the acquisition. That would be an instant deal killer. However, something tells me that they likely used the same language in reporting the metrics earlier as had been reported in the news post here. They take significant judgment but concede they cannot be 100% certain of the metrics... actually they state there could be "more" fake accounts. If that language was present during the initial discussion and since then Elon came out publicly to confirm an intent to acquire at a specific value.... then he can't use the user type variance whether in or against his favor to alter the parameters or back out of the deal without some serious potential cost to him.
Often times these sorts of deals have specific safeguards in place to protect each party. So there may be other superseding language in the contract(s). It's impossible to know without reading everything.
Either way, this could all have been handled privately or at least without another public announcement on his or Twitter's part. That is what makes this seem more like a performance than a genuine deal.
