It's not true at all. Even Erlang (very much not an OOP language) has public and private methods, with the default being private.

If you don't have a mechanism to indicate which parts of your software's API are internal implementation details and which parts are intended for use by users of that software, then you're going to have to rely on convention. The problem with convention is that no ten people do it the same way... which increases cognitive load... which is to be avoided in a field that has more than enough little things to keep track of already.