So what was the original saying? As I see it, this phrase is wrong no matter how you look at it. Because all ram is used at all times, for example if you have 32GB of free ram, the kernel will use all of it as a page cache to speed up the file system. The more free ram you have the more files can be cached, avoiding access to the disk when you read them.

This phrase is just plain wrong. Unused ram is used for the page cache by the kernel. You must always have some ram free or else the whole system will not operate without a page cache. Larger page cache allows to cache more files from the file system.