The IRIS-T (including the SLM version) uses an imaging infrared (IIR) seeker. This is very different from older heat-seekers that used simple scanning systems. An imaging seeker takes an actual picture of the target in infrared, which makes it much better at telling apart the real target from things like flares or laser interference.
Studies show that imaging seekers are much harder to fool with DIRCM lasers. A paper presented at the SPIE defense conference explains that when a DIRCM laser shines on an imaging seeker, it can dazzle part of the sensor, but the seeker can still see and track the target using the rest of the image. In other words, it’s harder to fully blind it compared to older missiles.
(https://www.spiedigitallibrary.org/conference-proceedings-of-spie/7115/71150A/Requirements-for-laser-countermeasures-against-imaging-seekers/10.1117/12.800244.short)
Defense research groups like FOI (Sweden) and TNO (Netherlands) have also tested how infrared seekers react to laser attacks. Their reports show that modern imaging seekers can resist a lot of laser dazzling, especially with extra image processing and filtering.
(https://www.spiedigitallibrary.org/conference-proceedings-of-spie/7115/71150A/Requirements-for-laser-countermeasures-against-imaging-seekers/10.1117/12.800244.short)
Important note: IRIS-T is not completely immune. Very powerful or advanced multi-band DIRCM systems can still cause problems if the conditions are right. But compared to older heat-seeking missiles, IRIS-T SLM is definitely much more resistant.
Conclusion for the game:
In reality, helicopter DIRCM would have a harder time breaking IRIS-T’s lock, but not an impossible time. A fair game balance would be to make DIRCM less effective against IRIS-T SLM, not totally useless, but not as strong as it is against old IR missiles.