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Discussing the question "Do you consider yourself superior to people in the old days?," poster Matthew here is looking for the title and author of a short story from a scifi anthology with this plot:

A man invents a machine that can grab a man from the future and bring
him back to the present. He uses the machine, and it works!

He speaks with the man, and sure enough, the society he comes from is
far more advanced. We didn't destroy ourselves or go back to the Stone
Age. All right! So far so good.

He asks how far Medicine has progressed. "Well, we've cured all
disease" the man from the future explained. "When someone is ill, we
just hold this little shiny cube up to them, and it heals them!"

"Tell me about this device. How is it made? What principles does it
make use of?"

"I don't know. That's something only specialists understand. I just
know that it's shiny, a bit warm, and 3x3x3 inches.

And the conversation goes on like this, in other areas of science and
technology: computers, communication, food production, warfare, etc.

Assuming that "Matthew" didn't remember any actual details about the story and just made up the part about the shiny black healing cube, it sounds a lot like "A Bulletin from the Trustees of the Institute for Advanced Research at Marmouth, Mass."a short story by Wilma Shore, originally published in The Magazine of Fantasy and Science Fiction, August 1964, available at the Internet Archive.

They grabbed a men from 2061 and have a short window of time to interview him before they have to send him back:

Q. You're located in the city? Suburbs? How do you get to work?

A. I catch the East express. I only have one change. It's about a nineteen-minute ride.

Q. On what?

A. The East express.

Q. I mean, does it go through the air?

A. How'd you think it went? On the ground?

Q. Well, but, is it a plane? A jet? You still have jet planes?

A. Only for locals. I take the express.

Q. And what is that?

A. I told you. The East express. Leaves seven thirty-nine, seven fifty-two, and eight sixteen. Then not till nine forty-eight. Seems crazy, bring a man back all these years and then not listen when he tells you.

Q. Well, you've given me so much interesting detail. Hard to take it all in. About the construction of this—express?

A. The seats are too close together. Your legs—

Q. And the overall shape of the thing?

A. How do you mean?

Q. How is it shaped? How would you describe it?

Q. Well, as a general rule, I just see the back end. I get in the left rear door; then when I go to change I'm right there at the local.

Q. What keeps it up?

A. Why, the machinery.

Q. It burns fuel?

A. Well, of course it does.

Q. You're being very helpful, Mr. Wencelman. Now, suppose you tell me what kind of fuel?

A. They keep saying they're going to invent something without fuel, but I'll believe it when I see it. And in the meantime, every year they raise the fare. Someone's cleaning up.

Q. A nuclear fuel, Mr. Welcelman? Can you tell me that?

A. I can tell you anything you want to know, just so you phrase your questions—so you phrase your questions, see what I mean? So I can understand them.

Q. Well, then, is this fuel a nuclear fuel?

A. Now, you know, I haven't been into all this since high school. If I had a little notice, instead of grabbing me in the middle of the night—

There's an astonishingly similar-sounding story called "Renaissance Man" by T. E. D. Klein. It also does not have the cube.

In this one a professor of "plasma biophysics" is pulled back into the past by a scientific team. But he can't explain how cancer is cured (though it is)

"That's a toughie, I'm afraid...You see, I've never had cancer myself..."

or how their ear-mounted phones work

"Gee...I'm sure I don't know..."

or their weapons

"You got me. I'm afraid I'm stumped."

It ends with one of the scientists saying "I guess even during the Renaissance, there weren't that many Renaissance men."