Amazon was wildly profitable on a unit cost basis from relatively early on. They didn’t show profits on paper because they reinvested everything into their capital buildout to reduce costs even more.

I thought Amazon was selling quite a few items at a loss to undercut competition early on?

They did in some instances, not all.

A notable example where they ate $ millions in losses is the Diapers.com story [1] [2].

[1]: https://slate.com/technology/2013/10/amazon-book-how-jeff-be...

[2]: https://arstechnica.com/tech-policy/2020/07/emails-detail-am...