King guided the Silkmen to a thrilling 2-1 win over their Championship opponents at Moss Rose on Saturday, with Matthew Barnes-Homer scoring twice in the final two minutes to secure a memorable comeback.
The Blue Square Premier side fell behind to a second-half goal from Cardiff striker Nathaniel Jarvis as Malky Mackay made 11 changes to the side which kept a grip on top spot in the Championship with a win over Birmingham on New Year's Day.
But Macclesfield fought back to reach the fourth round for the first time in their history and King reckons beating Cardiff was a massive result, regardless of who lined up for Mackay's side.
"We had them previously watched and had DVDs of them and Birmingham gave us match reports from previous games, it was a completely different side to what played on Tuesday," he told BBC Radio Five Live's Sportsweek programme.
"But, like I said to the boys, sometimes when you are playing against younger, hungrier ones they want to prove a point.
"They will be in the Premiership next year, all these players are on contracts, there are million-pound players out there playing, there are players who have played first-team games.
"The manager was still there and those players are going to want to prove a point to him, so sometimes it is harder to play against the young, hungry ones."
With Cardiff flying high and chasing promotion to the top flight they were the clear favourites to progress in the FA Cup but King reckons his side deserved the win.
"To be fair they had two chances probably in the whole game," he said.
"One that they scored from when they cut us open but we dominated most of the game and they had one long-range effort after they scored - I think we thoroughly deserved to win the game.
"This group of players, regardless of whatever happens this season, have made history at the football club, in 140 years they have never been in the fourth round of the cup so a little bit of history was made yesterday."
The result was one of the surprise outcomes of a memorable third-round Saturday with Barnes-Homer's late brace arguably the story of the day.
The 26-year-old striker has had a nomadic career that eventually saw him move to Swedish outfit Ostersunds in 2012 but King was keen to bring out the talent he had previously seen in the ex-Luton forward.
"I've known him for a while and known of him for longer, he has scored goals and I just think he lost his way a little bit, which players do from time to time," he said.
"It is up to you to identify that talent and can you nurture it and get the best out of it again?
"He is of a very good age and I sat down with him in the summer and kept in touch with him when he was in Sweden and just kept talking and talking and got into him in a way where he respects you and would give everything for you - I think he is the top scorer in the league now and has scored important goals."