From the same thread https://lists.gnu.org/archive/html/emacs-devel/2014-09/msg00...

> It's not a 100% drop-in replacement, that's true…(eq foo t) should always work. (eq foo nil) will only work if that `nil' came from Elisp code; if it's a Scheme #f or '() in disguise, then you need to make that (not foo) or (null foo) (both works in both cases); anything but (eq foo nil).