If you read the lesson carefully, it says "In this context [...] you can never omit the pronoun que." The specific context referred to is described in the preceding examples. Actually, there is a more general rule behind this:

When the relative clause and the main clause have different subjects, you need que.

Elle trouve qu'elles sont malpolies. -- The subject of the main clause is elle, that of the relative clause is elles.

Je trouve cela fascinant. -- There is only a main clause, no relative clause here. Je is the subject and cela the COD.

Hi Susan Colette,

The lesson is very comprehensive as it stands but perhaps you might like to refer to a quality grammar source to study Que as a relative pronoun and the verb Trouver.

https://www.wordreference.com/

Or Laura Lawless's grammar at lawlessfrench.com

Bonne continuation

Jim