I'm not so sure that you'd get that response. I will bet that there are a fairly significant number of people who have signed up for the Model 3, who test drive an S on a whim, who then get really excited and decide that they can't wait 2-4 years, and who then start looking at Model S 70s or at CPO cars.
Besides... the reps at the showroom are non-commission. And it looks good for Tesla to have a lot of people/excitement around the showroom. I encourage you to go take one for a ride. Now... I think I might tell the rep a slightly different story. I think I might say something along the line of "I've got a 3 on order, but I am seriously considering whether I want to wait 4 years and so I'm thinking about a 70.".
The bigger risk, I would say, is that you might end up turning your white lie into reality. My mistake was to take a ride in my friend's P85 - having no intent other than to go for a fun ride. Sure enough, 2 weeks later I was on the Tesla website pushing "Buy" on a P85D! Riding in an S is like a shot of heroin. Be very careful because you might not be able to control what happens next!
I'd be scared because I can't afford the Model S without jeopardizing my future. hahah I should disconnect my internet before I go for a test drive.