I reckon you must be right there: Certainly I've yet to feel even the slightest grab from any of the magnets I've tried so far. Of course, it's not possible to be definitive about the depth of the plaster but if tapping detects nothing, it must be fairly thick.
There seems to be a bit of smoke and mirrors about the way magnets are marketed though. While I'm sure the idea of 'Xkg pull strength' is useful for people wanting to stick a magnet to a steel plate and hang something on it, it's not very informative for a purpose like this. I'm not sure what would be a good measure, but it seems that it would be tricky to detect a weak pull on a heavy magnet.
If I could be sure it would do the job, I would be more than happy to buy something even a bit expensive, but the more I look into the idea of finding studs this way, the less likely it seems to be a good plan, despite being recommended by many sources online.