I'm honestly not sure if the sign says Zone 1 first (will take a close look next time and take a pic), however it stands to reason that they should be boarding business class passengers first, ahead of economy (regardless of status), this is part of what business class passengers are paying for. KL is basically saying FB silver and business class are equal when it comes to boarding order. Doesn't make much sense to me.
This is what it says on the website:
https://www.klm.com/travel/gb_en/pre...ents/index.htm
"Prior to the boarding process, we invite passengers with reduced mobility and families with babies under the age of 2 to pre-board first. They are directly followed by our Business Class and SkyPriority passengers.
At many airports, we use zone boarding. Your zone is stated on your boarding pass and depends on your travel class, fare type and frequent flyer level. Passengers in zone 1 are the first to board, followed by zones 2 through 5."