His two strikes - coupled with goals from Josh Walker and Andy Barcham - sealed a win which means the Iron are now five points clear of the relegation zone with six games of the season to go.
Robertson was only in the starting line-up because of a hamstring injury to Jon Parkin.
“I thought he was outstanding and in the second half I thought he was almost unplayable,” Knill said.
“I am so delighted for him. He played last week and didn’t have his greatest game and he was so disappointed afterwards.
“We were so disappointed for him because I see [this kind of] performance in him all the time.It’s taken a little while to come out but it’s there and today will help him no end and I thought he linked really well with Conor (McAleny) and they looked pretty sharp pair.
“I hope these goals will give him the encouragement to push on. They were both great finishes and he could have had a hat-trick right at the end but he couldn’t get his foot to it. He’s taken some knocks in his short career but when he plays like this, you can see there is a top player in him.”
Defeat leaves the Spireites bottom of the table, and needing to overcome a 10-point gap if they are to complete an unlikely escape.
It has been a bittersweet week for John Sheridan's side, who won the Johnstone's Paint Trophy with victory over Swindon at Wembley on Sunday but had relegation virtually confirmed with 4-1 defeats to Sheffield United and the Iron.
“It was poor on our part to lose 4-1 to a team we were hoping to make ground on, It’s very disappointing and we can’t accept that performance," said Sheridan.
“I will hold my hands up as to whether I am picking the right team or am I doing the job the best I can. I’ve just got to be strong and stand up for myself and carry on. I won’t be going anywhere and I’ll take what’s thrown at me and while there’s still a chance of getting out of the position we are in we’ll keep fighting.
“I thought we started the game really well and after 20 minutes I was pleased about the way we went about the game but after 25 minutes I thought we were very poor.
“We huffed and puffed but didn’t show any desire for a game that was vitally important that we won."