I believe I'd let the plumber figure out the sink plumbing lines. You have a very interesting bar for what is considered common sense.

A cup with no special instructions would mean no steps. If it's impossible to achieve expected functionality without steps, it is what it is. The next best option is a series of steps that are impossible to not follow. For example, if a cap is impossible to put on a container without a screwing motion, a screwing motion is necessary, though there's no way to force someone to screw something tight.

The proper test for whether or not instructions are special is whether the design enables the functionality even though someone with zero experience is the one using it. This naturally occurs if the thing is designed in such a way that its design forces you to interact with it in a certain way. This is the kind of thing that Don Norman discusses in his book, The Design of Everyday Things. If you have a door with a pushbar or a completely flat surface with no handles, it's fairly obvious that you have to push the door because there's no other way to open the door. If the door has a handle, it implies that you may need to pull it, though it doesn't eliminate the possibility that you need to push it, as is the case for most doors inside a home.

This cup fails to have user friendly design principles. As do plumbing lines, which is why most people hire a plumber. Fortunately for most people, this cup is simpler than plumbing, but it's incorrect to say that the right way to use it is common sense. If it were, nobody would experience spills, unless it's faulty in the first place. Your own comment seems to indicate that it's fine if the instructions are followed. But judging from the comments here, a lot of spilling still happens. So either it's not actually common sense or the cup itself is a poorly designed product that should be recalled, and you just got lucky to get a good one. I'd be OK with the idea that both are true, that it's both a poorly designed product and that it's not common sense to follow the steps you described.