From my understanding the middle seat policy does not exist.

There is a policy - short haul - to get Platinum and Gold (not Silver !) pax or higher fare paying non status pax to get to the front - but not if your booking contains non-Plat/Gold/high fare.

So we find ourselves often - Plat (for life) - my wife too - with 3 kids (Silver) in the last rows. KLM was surprised, shocked, apoliged and said the system was not working well. But with the number of flights we experience this, that did not hold - so in the end, they said only Plat/Gold/high fares and then only if the system works.

So in practice, we need to OLCI early and then call the Eliite line to get decent seats (together ) if at all.

In other words - KLM does not offer this benefit at all