There are better definitions. Monolith would be a full product (as we are talking Twilio, their main offering) in a single, tightly coupled codebase which does data passing mostly by directly calling appropriate code paths.

Service-oriented architecture is where you decouple parts of the full architecture into independent subsystems that communicate over opaque interfaces and without ability to break those boundaries.

Microservices architecture is a SOA taken in the direction of keeping them as small as possible, which can be very small!