In the design suggested by the article (and also used in places like Amazon) the L button should have been visibly disabled. I think this is pretty clear to mos

In the design suggested by the article (and also used in places like Amazon) the L button should have been visibly disabled. I think this is pretty clear to most people that it means that size is not available. It sounds like the issue you encountered was that the styling of the option didn't make it clear that it was disabled.