Derek McInnes' team were looking for their third consecutive win over Celtic but the home side were determined to show their championship form and won through doubles from Scott Brown and Kris Commons, who took his goal tally for the season to 30, along with a strike from Anthony Stokes.
When asked about talk of the League Cup winners closing the gap, Lennon said: "Well, we are 30 points clear. When we lost at Pittodrie I was asked the same question and if we had won the game that night, we would have gone 27 points clear.
"It's not for me to say the gap is closing. Aberdeen have certainly improved. I think they had to improve because there was a malaise at Aberdeen for a long time. Derek has turned that around and he deserves a huge amount of credit for that.
"With that there will be expectation and pressure next season, but for a first season he has done remarkably well. It's all down to him really. It's a similar group of players but he has certainly made them a lot more steely, which is something Aberdeen lacked for a long time.
"But they should have been challenging a lot earlier. For them to go 19 years without a trophy, speaks volumes for their underachievement probably more than anything else."
Brown netted a brace either side of a Niall McGinn equaliser with Celtic full-back Adam Matthews setting up all three first-half goals.
Stokes hit his 20th goal of the season but Shaleum Logan struck a superb first goal for Aberdeen and they had a strong penalty claim rejected by Iain Brines at 3-2 when Efe Ambrose appeared to bring down Jonny Hayes.
But Commons struck twice in the final quarter as Celtic took their post-split tally to 14 goals in three games, banishing hopes of an Aberdeen hat-trick in the process.
"There's been a lot of talk about it," Lennon said. "We have only lost one (Premiership) game this season and people want to keep reminding me about it and people keep talking about it.
"The players wanted to put a marker down and they have certainly done that.
"I thought we were great. It was five and it could have been a lot more. We gave Aberdeen a goal start too, it wasn't like they earned it. Our reaction was fantastic. We are playing very well and providing a lot of entertainment."
Brown added: "We knew we had slipped up twice but I think this showed how far in front we are and why we are champions."
McInnes is fully focused on this season rather than cutting the gap next term and the Dons are now set on sealing second spot after Dundee United's draw at Inverness saw them clinch a European place.
Motherwell are only one point behind though with their last-day meeting at Pittodrie looking like a decider.
McInnes, who lost striker Adam Rooney to an ankle injury, said: "It's two great games to be involved in. Dundee United at Tannadice is always a bit special for us and then Motherwell last game of the season could have so much on it, which would be brilliant as well.
"We will relish that opportunity to try to finish second."
When asked whether he thought Europe was a realistic possibility at the start of the campaign, McInnes said: "I wasn't really sure to be honest.
"I had only had a close season and a few weeks working, and you are never really sure how the team settles down.
"Maybe after the first quarter of the season, I felt it was certainly something we were maybe capable of. But it's a real positive to get into Europe, with two games to spare as well.
"Hopefully we can now go all the way and get second spot.
"There is plenty of motivation - professional pride to be next best after the champions, financially for the club there is a big difference between second and third, and also the players would get a couple of weeks' rest, along with myself."