How is this true?

The machine

It's been a week.

The two adults got the spare room that was once hidden in the wall. Stan explained that Soos found it a while ago while cleaning and the children had, in the old mans words, a hilarious fight about it. Of course the two already knew that, but they let the man explain anyway. On the first day of them being back in their original form, the two Stans let them borrow some of their clothes. But seeing as, apparently, Ford had only beige jeans, red turtlenecks and his lab coat, while Stan only seemed to own his stage suit and a way to old white shirt, they were going shopping the next day. Mark ended up forcing his friend to wear the suit because "I look way to much like Dark in this!", which he was absolutely right about.

After their shopping trip, Mark ended up with a loose pair of regular jeans, a black shirt and a red flannel. Seán got out of the store (the only one to buy clothes in all of Gravity Falls, which wasn't too surprising) with a pair of knee long jeans and the same black shirt Mark had. Both of them agreed that it was better than wearing the twins' clothing.

In the whole week, they didn't hear anything from Bill and his new friends. On the one hand it was very relieving and gave the two men time to adjust to these new circumstances. On the other hand it was more than worrying. The more time passed, the more time the three had to plan. Bill and Dark working together could make an incredibly horrible outcome, their smarts combined. With Anti, they made an even worse team. That man wasn't scared to get his hands dirty, and they knew it.

Currently, the two men, Stan and the young twins were sitting in front of the TV, watching a movie to pass time; only to get interrupted by a loud BANG! Now very much alarmed, none of the five were paying any attention to the TV, but rather armed themselves with the next best object. Mabel had her grappling hook in hands, where she suddenly got that from, no one was quite sure. Stan grabbed his cane from the ground next to the sofa, Mark had a lamp in hands, Dipper held his journal which was luckily thick enough to do quite some damage and Sean's hands found a knife. Very uncomfortable with his first decision, he looked over at Mark.
"Can we switch?"

"I did it!", Stanford yelled, marching through the door with pride steaming off of him.

"Boys, come on!", he said, dragging Mark and Seán by their arms down into the basement. Both dropped their weapons in the process.

"Sit down there!", the scientist instructed, pushing Sean into an uncomfortable wooden chair, once they reached his lab.

"Okay, I will connect this helmet to your head", the man explained, while holding up a metal helmet. Wires were coming out of it all over the place and on the inside, the two younger men recognised some parts of the alien tech they stole.

"These wires", Ford pointed at the wires coming out of the helmet, "are connect with this portal." The man pointed at the small, oval metal ring, which was just big enough so that one person could go through.
"I build that portal with the scraps of my old dimension portal. I changed some things, you probably wouldn't understand, and connected them with the helmet, which will connect with your brain. All you need to do is think about that... ego you want to bring here, and it'll work. Before the data goes to the portal, it'll go to my screen, so in case something goes wrong we will notice before the wrong person comes out. You got it?"

Both men nodded at the explanation. Ford went to place the helmet on Sean's head, but the man stopped him.

"Wait! Let me think who to get first!"