The points for seventh place were the first that Ricciardo, winner of eight career grands prix, has scored in four starts since making a comeback with the Red Bull's sister team last July.
They were also a strong statement after five races out with a broken hand and a contrast to Red Bull's Sergio Perez, the Mexican whose seat he may one day take and who retired after a first corner collision.
"You couldn't fail to be impressed by Daniel this weekend," Horner told Sky Sports television. "His qualifying was outstanding yesterday and he showed his maturity and experience and pace in the race.
"I think fighting a Mercedes in an AlphaTauri for their best result of the year was a great performance. Had it not been for the red flag he might have actually finished even further up."
"He can take a lot of confidence coming back from injury, he's had a rough time over the last couple of months having to sit on the sidelines but that's the Daniel we're used to seeing."
Ricciardo had qualified fourth on Saturday at the Autodromo Hermanos Rodriguez and told reporters that performance was no fluke.
The points lifted AlphaTauri from 10th and last in the standings to eighth.
"His confidence is coming up," said Horner of a driver who was at a low ebb after difficult stints at Renault and McLaren after leaving Red Bull at the end of 2018.
"Checo (Perez), we just need him to have a confidence-boosting result. He could have had it today but it wasn't to be but great to see Daniel looking like his old self."
Ricciardo said he felt relieved and "probably better than good" with a day that involved two standing starts after the race was red-flagged before the half-distance due to Kevin Magnussen crashing his Haas.
"When I saw the red flag I knew it was probably going to hurt our race, we were in fifth and potentially could have finished there," he said.
"But six points, you really can't be sad."
