This documentation is for the Upsy Desky, not the original WiFi Standing Desk Controller. You can tell which one you have by looking at the port it uses for flashing. If it's a micro-USB port, you have the original WiFi Standing Desk Controller. If it's a USB-C port, you have the Upsy Desky.
Firmware for the original WiFi Standing Desk Controller is available here.
If you don't have ESPHome readily available, you can flash a pre-built firmware to your Upsy Desky. These binaries will advertise a captive portal on an ad-hoc WiFi network so you can connect it to your WiFi without needing to install ESPHome.
If you are using a supported browser (i.e., a Chromium-based browser), then you can flash the firmware directly from the web. Press the blue button, connect the Upsy Desky to your computer, select it from the list, and install.
If you are not using a supported browser, or you need the raw firmware binaries, you can download them from the GitHub releases page.