“he can just claim that he will fulfill the requirements after death.”
Exactly! Why would you think otherwise? Now that Spinoza has died and having been faced with the truth, of course he is waiting for his chance to return to life. He will then to a complete teshuva, marry and have many sons. Obviously, he is the leading candidate for being mashiach. Many, many, great talmeidi chachamim wrote about spinoza. None of them ever said that he will not be moshiach. And if your one of those types that ignores all the sources in favor of the Rambam, the Rambam doesn’t explicitly exclude those who are resuscitated from among the dead. The Rambam couldn’t explicitly say that it would be someone who comes back after death, because Spinoza wasn’t born yet. And there was the possibility of him becoming mashiach before he even died.