The actor spilled the tea once and for all in a new interview with NYT -- saying this about his and Sydney's movie "Anyone But You" ... "The two things that you have to sell a rom-com are fun and chemistry. Sydney and I have a ton of fun together, and we have a ton of effortless chemistry."

He adds, "That’s people wanting what’s on the screen off the screen, and sometimes you just have to lean into it a bit -- and it worked wonderfully. Sydney is very smart."

Sydney herself also chimes in in this same interview, confirming she was the one pulling the strings on getting everyone to think she and Glen were an item -- which fueled a lot of dating rumors and gossip ... but which all ended up being completely bogus.

She says, "I was on every call. I was in text group chats. I was probably keeping everybody over at Sony marketing and distribution awake at night because I couldn’t stop with ideas."

Sydney continues ... "I wanted to make sure that we were actively having a conversation with the audience as we were promoting this film, because at the end of the day, they’re the ones who created the entire narrative."

You might recall ... Glen and Sydney shared lots of lustful stares while promoting that film, and they certainly looked cozy as they flirted on camera -- which pretty much lasted through the entire press run. Of course, everyone ran with the notion they might've been banging.

It certainly helped that Glen's real-life relationship just so happened to fall apart at the time ... even though Sydney's engagement to Jonathan Davino is still going strong.

At the time when this was all unfolding, we had some experts tell us their off-screen chemistry was completely contrived and nothing but a PR stunt -- and as it turns out ... they were correct. "Anyone But You" did relatively well at the box office -- so we all got duped.