Brian Acton, of WhatsApp fame, joined with the creation of Signal Foundation circa 2018. Moxie Marlinspike and others created the software that became Signal much earlier, they were not part of WhatsApp.
Also, it's worth noting that the Signal team didn't work on WhatsApp. It was WhatsApp that chose to integrate the Signal protocol, which Open Whisper Systems/The Signal Foundation open-sourced their protocol.