...except in the case of an en bloc clip such as used in the M1 Garand, which goes into the magazine and is itself part of the feed mechanism.

My pet theory is that the M1 is a big source of the current conflation of "clip" and "magazine". Millions of men were conscripted for WW2 and Korea in the US and for a great many of them their first experience with a self-loading firearm was with the M1 they were issued in training. To many of them, no matter how many autoloaders they were exposed to aftward, that thing you shoved into the gun that held the cartridges was called a CLIP, because that's what the drill sergeant in training called it. These men then went on to spread this habit of using CLIP and MAGAZINE interchangeably, and at this point it is too ingrained in the vernacular to ever go away.