He mentioned the Hondata intake manifold insulator gasket which goes between the head & the intake manifold which includes a coolant passage.
He will have to drain the coolant, open up the drain on the rad & let it drain into a bucket. Then remove the intake manifold, clean the mating surfaces, no need for silicon sealer or any other gasket sealer.
Then refill the coolant, (I would recomend using new coolant), run the engine & bleed it. Jacking up the nose of the car helps to bleed the air out of the system thru the rad cap.
If you mean the insulator gasket between the upper & lower intake manifold, there are no coolant passages involved there.
you never ever put silicone on a img or for that matter any gasket the rule is either or. you use a gasket or silicone sealer. i have been a honda master tech for 15 years and thats a no no. i hate when a car comes in with that all over the gaskets