Orient went into the game knowing they needed to at least match Reading’s result on the final day to ensure they held on to sixth spot.
And while the Royals went down at home to Barnsley, Orient now have a two-legged semi-final against Stockport County to look forward to.
Wellens said: “We just needed to concentrate on ourselves. I thought in terms of our counter-attack and our speed in transition, we thought they would press us high, but with the speed we’ve got in our team we could break through them.
“We probably could have done more, but I have to say Huddersfield have some good players.
“I’ve been in that position myself and when you get close to play-off semi-finals and play-off finals you start thinking about the next game.
“We knew we had to just stay in the moment.
“We actually watched Luton against West Brom before the game and some people might think West Brom have got nothing to play for – it doesn’t work like that.
“Professional footballers are paid to do a job and we expected to come here for a tough game.
“But I thought we were at it and when we’re at it we’re dangerous. Nobody expected us to be here, but here we are.
“How we’ve not got a player in the League One team of the year, I don’t know.”
Dan Agyei scored twice for Orient, who also netted through an own goal from Huddersfield goalkeeper Jacob Chapman and substitute Azeem Abdulai.
The hosts gifted Orient the lead within two minutes as Josh Koroma’s mis-hit pass played in Jamie Donley to run unchallenged into the box, where a square ball gave Agyei a simple finish.
Agyei went close to doubling the advantage five minutes later but hit the crossbar with Jacob Chapman well beaten.
The Huddersfield goalkeeper then made a mess of a deflected cross from Agyei, flapping the ball into his own net for 2-0.
Josh Koroma pulled one back just before half-time with a looping header into the top corner and Orient had assistant manager Paul Terry sent off moments later for preventing a quick throw-in.
But Agyei made sure of the result shortly after the break, once again applying the finishing touch to a swift and clinical counter-attack.
And it was a familiar story for 4-1, substitute Abdulai converting a close-range header to finish off Ethan Galbraith’s cross on the break following a Huddersfield corner.
Huddersfield caretaker boss Jon Worthington said: “It’s been extremely challenging [being in charge]. Probably in hindsight I walked into something that was a lot more challenging than I thought it would be, if I’m being honest.
“I’ll reflect on it over the summer and really take stock of what’s happened in that 10-game period.
“What I can confidently say is I can look at myself in the mirror and definitely know that I’ve given everything I’ve got to try and help.
“I don’t want to dive too much into the detail, but I think people who have watched the games since January, it’s clear to see what some of those challenges and frailties are that we’ve had in this period.”