I noticed that Insightly allows users to sign in using their Microsoft Office 365 credentials.
Do I need SAML, which is a feature on Enterprise plans, to use the Single Sign-On feature with Office 365?
No, you do not. SAML does not need to be enabled for users to sign in with their Microsoft credentials.
You only need to use SAML for features such as multi-factor authentication, integration with Okta, PingIdentity, OneLogin, ADFS, etc.