You are correct. Paul uses that term in v 20 to refer to utterances or teachings that reflect divine revelation. He's telling the Thessalonian Christians not to despise such prophecies but to instead test them.

Today, we have many people who (with the wave of a hand) dismiss (or "despise") the very idea of a supernatural message - a divine revelation. Religion is entirely a man-made construct, they say. God is a figment of our imagination, they say.

These are people in violation of I Thessalonians 5:20 today.

However, going to the other extreme, there are those who never test anything. They believe the most ridiculous stuff, and never let any of their beliefs be tested. These people are in violation of I Thessalonians 5:21.

I would rather not violate either verse.