The few times they've had to lip synch since their return has been for technical reasons and this was probably one of those times.
They did sing the single live a couple of weeks ago. Besides, everyone knows Gary can sing and TT are one of the best live acts, so it's not like they had something to prove on that front.
It was a fun throwback to the 70s and a bit of tongue in the cheek on the part of the boys. It's true that the performance targeted more the hardcore fans, but anyone with a sense of humour should have enjoyed it.