Windows considers it the same as being in a small window. Borderless is just a window with no frame around it, sized to the whole monitor.
The difference is that when a game is running in true fullscreen, Windows gives it more direct control ('exclusive mode') that prevents anything else from using some parts of Windows/your hardware. When you alt-tab out of a true fullscreen game, Windows is taking that control away from the game, giving it to Windows, and then passing it back to the game when you go back. That's where a lot of games (including most Bethesda/Creation Engine games) start having problems, as they can't handle control being taken way.
But since Windows sees borderless fullscreen games as running in an actual window, it never activates exclusive mode, and the game doesn't have to recover when you go back. When you alt-tab out of a borderless fullscreen game, you're just switching between programs, not turning parts of the game completely off.
In fact, that's the biggest benefit to borderless - switching windows. Most (not all) games actually run very slightly better in true fullscreen, but alt-tabbing (or switching monitors) is slower and/or problematic.