This is the same for almost every Mac/ iOS email client....Spark, AirMail, Canary etc.Outlook for macOS (like the iOS version) stores your account credentials and mails on Microsoft servers (also the credentials an mails from non-MS accounts). It‘s a privacy nightmare. If you care for privacy I highly recommend you: don‘t use it!!!
I‘m a little disappointed that macrumors doesn’t mention it in the article.
They are free because they download your email from your provider's email servers to theirs, which is what enables all the extra cool functionality, as well as the ability for them/ AI to read your emails.
This has some interesting consequences depending on where that company is based. In the case of AirMail - Italy. Spark, in the Ukraine....I would never be able to use these clients with my work account, even though they are technically compatible with our Exchange servers. Offloading credentials/ email to a third party server is a big red flag.
The big difference with MS is my Gov. Employer has their services provided by MS anyway so my email never goes to a third party and the Outlook Apps are fully supported from a governance standpoint.