They link to an old article [1] that was featured in HN [2] somewhat recently, in which there's a workaround for older standards with regards to typeof.

[1]: https://danielchasehooper.com/posts/typechecked-generic-c-da...

[2]: https://news.ycombinator.com/item?id=44425461

It is a GNU extension, supported by both GCC and Clang.