The phrase " Elevator Pitch " is an old phrase relating to a very brief time period in which to " pitch " or present a product or project to a potential client or buyer.
The use of the word " elevator " means the short time you might have a person's undivided ( and usually involuntary) time and attention, as if you and the prospect were in an elevator riding between a few floors together and you - the seller / presenter - just " pitches " the product/project.
Elevator Pitch for design thinking = Very quick, concise sales pitch re a proposed design, IMO.