Gassing means lactic acid that makes it hard to grapple or throw strikes, slows you down. It doesn't mean you have to be physically tired, but that can be a part of it.
Reem didn't show any of this, but the opposite: he toyed with BF for two rounds, then went forward and tried to end it at close range, also with knees from the thai plum. He got caught while trying to do so, and never recovered.
Overeem's cardio at HW has been better than at LHW, but it's still average at best even for HW standards. This Silva fight was also slow-paced, with Silva doing next to nothing and Alistair picking his shots for the most part. That also helps to prevent gassing.
WFA never die