Well playing devils advocate, it sure wouldn't catch any alluminum, and as far as I know that engine is mostly if not all alluminum. I do however have one on my 1970 Camaro that has a huge cast iron block. And on each oil change I seem to be able to pull some iron bits and pieces off of the magnet. Whether or not it would make much of a difference I don't really know because it would have been caught in the oil filter anyways. This may be an interesting thread.
I'ts my understanding that the block and head are aluminum, but I'm sure the cam and crankshaft and oil pump parts are not aluminum, and particules form these components and others would be picked up by a magnetic oil drain plug.
So from where I'm sitting I don't see any egg on your face...
Most of the moving parts in an engine, aluminum or other wise, is steel. Those are the parts that wear, those are the parts that need to be watched. I use magnets in engines more to monitor there condition then as a filter. If your antique engine doesn't have a filter, a magnet may be your only defense against debris. I have one in my antique airplane, but I don't use one in the Insight.
1. The insight engine is sleeveless.
2. Crankshaft, con rods, camshaft and lots of other moving bits are made of steel. (i.e. magnetic)
3. The thing I'd be most interested in the wear state of are the bearings. They are usually made of metals like brass, lead, tin, AL and other common alloys. (i.e. not magnetic)
Bottom line. A magnetic plug will not remove anything from your engine oil that the oil filter does not remove. It might give you an indication that your engine is having abnormal wear. But if you do find abnormal wear, what are you going to do? Honda probably won't fix anything under warantee till it breaks.