The Belgium star netted a clever free-kick, slotting under the jumping Cardiff wall after eight minutes, before Raheem Sterling headed his 19th goal of the season.
City were rarely threatened thereafter and De Bruyne reported Pep Guardiola's tactical plan worked like a dream.
"They've a different playing style to everyone else. They almost play man against man across the whole pitch. It's a little bit weird," he told BBC Sport.
"The tactics we used were class because Vinny [Kompany] was always free and we opened up the midfield.
"We had a lot of space. The second half was strange because they didn't want to attack us.
"I think it was a little bit boring for half an hour but we could save energy for the next games."
Those are contests, starting with the Premier League visit of West Brom on Wednesday, that are likely to take place without Leroy Sane.
The Germany winger was scythed down by Joe Bennett, the Cardiff full-back who was later sent off for another ugly challenge on teenage substitute Brahim Diaz, and did not emerge for the second half.
An angry Guardiola told reporters Sane would be assessed further on Monday and could be out for up to a month.
"We knew how they played," De Bruyne said. "They caught us a few times and that is football but obviously Leroy is injured now and hopefully that is not too bad."
5th round, here we come! pic.twitter.com/7nbj1WtyF8— Kevin De Bruyne (@DeBruyneKev) January 28, 2018
The opening goal also arrived as a result of a Cardiff foul, even if De Bruyne's ingenuity and attention to detail in preparation were the primary factors.
"They [Manchester City's coaching staff] tell us before the game. We know at all the details," he added.
"It was a guess. The wall was very close so it was easier to put it under than over."