Their mistakes is what saved Snowden because otherwise he would be in prison now.
Source: The Putin Interviews by Oliver Stone
imho 'is' refers to the noun 'what' that is from the right side of the verb 'to be' that is quite unusual for me. Why does this verb take a form regardless of the noun that precedes it? Is it possible to use 'are' in this case?
ps I've seen the similar topic - Referring to a plural noun as singular? But it didn't help me to understand the usage