IR seekers have very (in air-air combat terms) short effective range, ~25km at best. Thus, a missile with an IR terminal-stage seeker needs to either be launched from close range (which would indicate the F-35 was probably detected via Mark 1 Eyeball) or the missile needs to be guided into range by radar. Guiding the missile that way involves painting the target with a fire control radar and having a passive radar seeker on the missile follow the return. Reduced radar signature obviously helps in the second case.

Edit: fixed seeker range

IR is only really effective in the short-range (see an aim-9 which is only effective less than 20 nm (really less than that practically speaking). When they talk about this, they mean protection against beyond visual range missiles.