He said that because it was obvious that the USA would impose an embargo on the island and force most of the world to follow it, it was Castro's fault. He said that if Castro had his people's best interest in heart, he would have caved against american pressure.
I don't even think it was obvious at the time. Lots of countries tried to have a non-aligned anti-colonial movement early in the Cold War, and I don't think they expected, initially, that America would reflexively try to destroy them.
Ho Chi Minh quoted the American Declaration of Independence in the Vietnamese Declaration of Independence when he was rebelling against the Japanese occupation ... after making contacts with the OSS (aka baby CIA), which thought early on that he might be their guy in Indochina. People thought that America could be appeased.