The Lua 5.3 function luaL_checkversion makes sure that there is only one Lua library linked to an extension module, and that this library has the same version that was used during compilation of the extension module.

The compatibility implementation for Lua 5.1 is empty, so it doesn't do either of those checks. It simply makes your code compile if you use this function in your extension module code. Lua 5.2 is not affected and is fully compatible with the version in Lua 5.3.