Does anyone know whether HopMobile really does roam in Cuba? Cuba is listed on their site, but when you look at the costs other operators charge for roaming there, Hop's rates sound almost too good to be true.

I don't know.
However, you could also say that Hop has too good rates also in the US, Canada or India to compare with other international SIMs :)
They just have the pricing policy of unified rates worldwide and this feature results in overpaying in some countries (e.g Europe) whereas enjoying good rates in others...