The Law does not deal in Thesaurus/Simile English.
This is like saying possession of purloined goods isn't illegal, because it's just a prettier word for stolen and isn't in the statute.
Giuliani has been out there, for months, saying there was no campaign collusion and it's not a crime, even if there was. Now that Manafort was convicted and shown to have given campaign data to Kilimnik, he's changed his tune to say...
“I never said there was no collusion between the campaign, or people in the campaign. I said the president of the United States."
Notice he completely abandoned the technical aspect and never said any potential 'collusion' on a campaign member's part is irrelevant to law. He knows that would be roundly and rightfully mocked by people on all sides.
The word 'collusion' may not appear is specific statute, but conspiracy does. In that context, they're completely interchangeable. You cannot engage in a conspiracy without colluding. Investigation of 'collusion' doesn't appear in the SC mandate, anyway. So, your attempt to split hairs on definition and synonyms is kind of senseless.