The seat map is not a good way to determine availability. Many pax have not selected seats and KL may, for one reason or another, have blocked seats.

Most carriers overbook. Oversell cannot occur until there are more passengers properly checked in and timely at the gate ready for boarding. Carriers are pretty good at overbooking enough to ensure full flights without going too far and creating an oversale.

If a cabin is oversold or at risk of oversale and there are seats in a premium cabin, KL will try to sell those seats as upgrades and then, if necessary OPUP to create space. Then and only then will it turn to denying boarding, either through volunteers, some of whom may jump at the chance not to travel, or involuntarily with EC 261/2004 compensation and rebooking.

Actual involuntary denied boarding should be rare and only occur when aircraft substitutions or broken seats throw off the calculus.