Cullen and Leinster, who had comfortably cruised through the tournament last season, came under intense scrutiny after Northampton’s fast start saw them take a 12-point lead at half-time.
The four-time champions fought back and almost snatched the game at the death, with Cullen revisiting late refereeing decisions that cost his side in the 37-34 defeat last May.
A year on, he defended his side, who had previously never beaten three-time champions Toulon, and said he always expects a Champions Cup semi-final to “never go to script”.
“Naturally [in the last 10 minutes] you try and protect things, don’t you? Whereas the other team they don’t have protection and throw everything at it,” he added.
“We were sitting in this room this time last year. We were in that situation and we were throwing everything at Northampton. [Henry] Pollock gets a poach – it should have been a penalty.
“It’s clearly illegal, but nobody wants to report about it after. We should have had a penalty try and nobody wants to report about it. You just want to kick the boot into us, don’t you? But that’s the way it goes.
“Semi-finals come down to the tightest of margins. In 2012, Wesley Fofana knocked the ball over the tryline and that is how we [Leinster] got to the final.
“I would be kind to Toulon as they showed great spirit to the very end.”
Leinster will travel to Bilbao for the final on Saturday, 23 May, where they will face last year’s champions Bordeaux-Begles or Bath, who play on Sunday.
Cullen confirmed centre Robbie Henshaw and flanker Josh van der Flier, who left the field with head injuries, will undergo the graduated return-to-play protocol.
Flanker Jack Conan limped off and “will get checked”, with Tommy O’Brien said to be likely suffering from cramp.
All four are important players for Cullen as Leinster seek a fifth Champions Cup in three weeks’ time.