What does 100% Paragon or 100% Renegade actually mean?

The ending that you're referring to doesn't require you to have 100% Paragon or Renegade, but rather

that you have used a Paragon or Renegade conversation option on him at every other opportunity.

From the Mass Effect wiki:

There are several conversation branches with associated Reputation checks, but the conversation's outcome hinges solely on the final choice. In order to use the final Paragon/Renegade conversational options with the Illusive man, you must have used a Paragon/Renegade options on him at every other opportunity in the campaign, including the meetings on Mars, Thessia and the Cerberus base (which were discovered through other dialogue options.

I managed to convince the Illusive Man to commit suicide, and I'm definitely not pure paragon.

Either your sources are wrong, or "pure paragon" doesn't mean what you think it means. (Maybe always do Charm when talking to the IM, and not swap between charm/intimidate?)