Webpack existed 10 years ago, and that was after Browserify had already existed for a bit, too. The era of bundling was already in full swing in 2015, and React gained support for ES Classes, too.

React wasn't really terribly usable without bundling. Prior to bundling, there were multiple competing adhoc module systems, but obviously this wasn't fantastic for a variety of reasons. AngularJS 1.0 was great in part because the dependency injection system kind of took care of a lot of trouble; you could have a workflow in Gulp/Grunt concatting your JS together, and the order didn't really matter a ton since dependency injection would largely handle it.

React moved relatively quickly in the first few years, but 10 years ago was already not the very beginning of React. It was starting to mature by that point.