Hello novicelectric. Welcome to the Community!
Great picture! This really helps.
The Defiant timer does function as a true 3 way switch with one important difference. This timer requires a neutral wire in addition to the 3 “power” wires a switch needs.
Your diagram shows NM-B or “Romex” brand cable being used rather than loose wires running through conduit. Because of this, the white wires that run to the switches are used not as neutrals but as power wires. That is why they are correctly marked in the diagram with those black spots near the switch and wire nut connections. The only true neutral wire in your diagram is the white source wire that goes directly to the lamp.
Not having a neutral wire going to where you mount the timer will be an issue, as the timer will not work properly without it, and adding a wire into a system of cables is no simple task.
The “jumper” wire seen on the 3-way switch side is easily added, as it only exists inside the electrical switchbox. Below are the 3-way wiring options from the Defiant installation instructions.
Thanks, Chris.