It's a great question to ask.
often Dachshunds have a general, no fear type attitude even in the face of injury. It's not suggested nor good for them to jump off furniture, beds, chairs more than there body length. is it hard on there backs "absolutely" and the effects are not always noticeable while they are young unless they are subjected to personal injury early on in life. It may be cute now, however they will pay for it later on in life.
My little girl sets up with out me teaching her at all so that's a Consent "no, no" I have to work with her on. My little boy however doesn't sit up. but like yours he's a mid air jumper right off the back of the chair, couch or bed. He's healthy and very strong but I know he'll pay for it in bad back, and shoulder and joint problems later on in life.
will they do it the moment your not looking "yeah" ... but that's what I say also "no jump" Do they get stubborn like they don't hear you "yeah but stay on it" They like to have selective hearing even though they understand you perfectly.
All long term Dachshund owners like many large breed dog owners know it comes with the territory with specific breeds of dogs, and digging, jumping, potty training and general stubbornness comes with the territory with Dachshunds.
Don't make your self and your Doxie unhappy by panicking to much. try to minimized and discourage it as much as possible for there general long term health.