This study is flawed for a few reasons that I'd mentioned to the author when it was first published, but they ignored my comments.

The main problem is that they confuse DOMInteractive for Time To Interactive. The two events are very different. DOMInteractive is a legacy event that refers to the time when JavaScript can safely interact with the DOM. This really has nothing (or very little) to do with user experience.

Time to Interactive, on the other hand, is the time when a user can expect the page to respond quickly to interactions (clicks, scrolls, keypresses), ie, these user interactions should have no user perceivable latency. This is not a metric that has a corresponding browser event. It requires a bunch of post-processing based on various events. The researchers have made no attempt to calculate TTI.

Source: I've been working in this area of research for 15 years.