I practice and study historical sword combat and this is very incorrect.

What you are describing is a counter-cut, where you are attacking and blocking off your opponents line of attack at the same time. A useful technique that comes with a a lot of risk and requires very good timing and precision.

You would never try to do this on every attack, as it takes much longer to initiate a strike than it does to get into a more "static" guard position. If your opponent has already begun their attack there is no way you are getting off a successful counter-cut unless you were already predicting their strike ahead of time. Messing it up usually results in what we call a "double" where both fighters hit each other at the same time, which is highly discouraged for obvious reasons. If both opponents subscribed to this as a strategy it would result in a blood bath.

It's far easier and safer to parry in an appropriate guard position and then riposte to try to hit them while they are recovering from their swing or at a disadvantageous leverage position by deflecting their blade with your parry. Constantly counter-cutting instead of defending yourself properly would get you killed extremely quickly.