Being underestimated is the one time nobody bothers guarding you. Take the open lane.

Low expectations come with real freedom attached to them. Nobody is watching closely, nobody is competing hard against you, and very little you try carries a risk of public failure. That is a genuine advantage, though it only stays one if you use the space instead of arguing with the assessment.

Stop trying to correct the estimate and go do something with the room it gives you. Prepare more than the situation appears to call for. The moment it stops being true will announce itself, and arguing in advance has never once sped that up.