Managing personal Outlook.com account settings, security, and privacy
Am I to understand that Microsoft has not built in any measures to prevent people from reading any correspondence I might leave on the Word Online site?
I think it's quite a stretch going from what JMH3143 said to the conclusion you're drawing. Naturally Microsoft has built in measures to prevent this, but no security measures are 100% secure. The only way you can reasonably guarantee that no one can access your documents is if you create them on a computer you never connect to a network.