Is "cast in stone" a malapropism?

There'a a similar question (How can something be "set in stone"?), but "cast" is a more specific verb than "set".

I was telling about Russian PM's famous malapropism "my words should be cast in granite" to an English speaker when I realized that "cast in stone" sounded rather natural. I checked, and this expression was present in both Collins and Macmillan dictionaries without any remarks about its usage.

You can cast something in metal, plaster or concrete, how did this verb stick to 'stone'? Is 'cast in stone' really a valid expression?

There is a valid usage of cast in stone: when stone is used as a mold for casting something else, such as lead ingots.

But otherwise, "no" cast is misused when what is meant is set/etched/carved.

However, between 1970 and 1990 (according to a time-bracketed google book search), the "incorrect" idiom seems to have suddenly sprung up in literature.

At this point, the idiom itself seems to be generally understood and passes without comment (present company excluded).