Condescension is not usually about tone. It is about scope. It happens when you answer more than what was asked, or correct something the other person didn't get wrong for the purposes of the decision at hand. Someone asks whether the deadline is realistic and you launch into the architecture, the vendor's history of missing SLAs, and a tangent about a project from 2019. You were accurate on all of it. You still made them feel small, because you answered questions they never posed.
The discipline that fixes this is separating positions from interests. The position is the question they asked out loud. The interest is what they actually need to decide or do next. Most of the time the interest is narrower than your full knowledge on the topic, and condescension is what happens when you answer with everything you know instead of what the decision requires.
Mirroring helps you find that boundary. Repeat back what you heard before you answer: 'So you need to know if this ships by the 15th.' If you got it right, they confirm and you answer that, precisely. If you got it wrong, they correct you and you just saved yourself from a ten minute answer to a question nobody asked.
You are not dumbing anything down. You are matching the size of your answer to the size of the question. That is not simpler, it is more disciplined, and it is the difference between sounding like you respect the room and sounding like you're using it as a stage.
Mirroring confirms what was actually asked before you answer it; separating interests from positions keeps your answer sized to the decision, not to your full knowledge on the subject.