There are large differences between outside running and running on a treadmill.
When referring to running for speed on the treadmill, yes, a percent or two incline is suggested to make the metabolic effort more similar to outside since you aren’t dealing with air resistance.
When running outside at say 6:00 mile pace for a repeat, you can be running the same exact PACE indoors with less actual metabolic cost due to not having air resistance contributing to your effort.
However, for practicality you have to take some things into consideration. Is your treadmill actually reporting the correct speed? Is your indoor environment actually more stressful than outside due to increased temperature and lack of air circulation? How is the belt responding to you landing on it? Is the motor keeping up or is it slowing down after working harder for a while?
I’d disagree with other comments saying you have to put an incline due to form reasons, but to each their own. Whatever gets you through mentally on the treadmill is going to be your own personal % advantage.