No. Thermal guided weapons can intercept a target not previously locked by IRST if it is fired in predicted target path in such a way for seeker to pick up said target on its own line of sight.
there was a tacview video of a guy who performed cobra under target incoming, fired an IR missile at the outro of cobra just as he was getting level again, the missile circled high above him and when seeker picked the target behind him, it followed.