Unless it's for a position where that's the standard, e.g. certain investigative journalism roles, I'd call it a red flag and have the scam sirens blaring. Although as a cybersec professional I'd be VERY curious to see where it goes and what their deal is. I'd MAYBE expect some security roles to request E2E encrypted email comms at a certain point in the hiring process depending on the project, there shouldn't be anything discussed pre-onboarding that necessitates using Signal.