The most telling moment of the U.S. antitrust trial against Meta so far came halfway through more than 10 hours of testimony from Mark Zuckerberg, the company’s chief executive.

On the witness stand last week, Mr. Zuckerberg, who founded Facebook and later renamed his company Meta, was asked by government lawyers to watch a seven-minute video of an interview he gave at a tech conference more than a decade ago. With his brow furrowed and eyes scrunched, the 40-year-old tech billionaire watched his 28-year-old self describe how the world of 2012 had “really underestimated” his company.

Back then, smartphones were a burgeoning computing platform rather than the dominant one. Facebook was still primarily used on desktop computers, and Mr. Zuckerberg’s social network was at risk of losing users to a flurry of start-ups.

The older Mr. Zuckerberg occasionally winced as he watched his younger self on video discuss some of his early competitive concerns, such as the potential for Dropbox, a file-sharing company, to become a rival in photo sharing. In hindsight, he said on the stand, it was “pretty ridiculous” to think that Dropbox would compete with Facebook.

It was, in short, a reminder of a very different era — an age of social apps, Silicon Valley hubris and Ivy League-educated entrepreneurs, which Meta’s antitrust trial has plunged people back into, in a courtroom in the U.S. District Court for the District of Columbia. At issue in the landmark case is what could have happened if Mr. Zuckerberg had never made two key deals, buying Instagram in 2012 and WhatsApp in 2014, and whether he quashed competition with the acquisitions.

Sitting in Judge James E. Boasberg’s courtroom for the first two weeks of testimony has been like entering a time warp. Executives who have long since left Meta — including Sheryl Sandberg, Mr. Zuckerberg’s former second-in-command, and Kevin Systrom, a founder of Instagram — are main witnesses. Lawyers have dredged up emails that date back more than a decade.