I suspect it may be complex for a complete beginner.
The PCB itself is fairly simple. There are 10 buttons that need to be soldered in. These buttons connect to specific pins in the 2x20 pin array that form the standard Raspberry PI pinout. The standard Teensy pinout is completely differ.
You would have to translate the Pi code to Teensy code, and map the Pi pins to the Teensy pins and then solder in jumper wires to connect the two sets of pins.
This is where the main tricky part is. Ideally, you should be familiar with both programming on the Pi and programming on the Teensy to do this transformation. And if you want to do graphics or audio it will be even worse. Things are completely different between the two systems. It can be done, but essentially, you will be completely completely rewriting the program, using the Pi program as a template. And note, the LC has very little memory compared to the Raspberry Pi. It may be the program is too large for the LC (or maybe it will fit just fine).
There are 4 additional connections for the audio. While the Teensy LC has a DAC (digital -> analog) port that can be used for audio. Unfortunately the Teensy LC does not have support for the Teensy Audio Library. If you wanted to play simple sounds there are ways to do it on the Teensy, but ideally you would want to step up to at least a Teensy 3.2 if you wanted to play audio.
Speaking of audio and graphics, the Raspberry Pi has a filesystem on its micro-SD card, and programs generally keep the graphics/audio files there. The Teensy LC does not have a micro-SD card, but you can attach one if needed. But that is more programming.
What the PCB does is just give you buttons in the position of a different controller. It may be better to not worry about getting the shape of the controller right immediately. Instead, put the buttons on a breadboard, and use that as you are coding. Then when you have the program working, you can integrate the PCB.
Or possibly step back, and take stock of what you want to do. Maybe using the Pi will do what you want faster than the Teensy. Maybe doing something else on the Teensy will work. You can keep using the PCB as the end goal, but you want to have smaller achievable steps in between.