I agree that unless the vehicle is being driven under "harsh conditions" (as described in the service manual) the service should wait until 15,000km but the part about single weight 10w as a factory fill is pure conjecture on the Service Advisors part. For a start, it wouldn't be 10w(inter), it would be just 10.
Re: your comment Castrol = genuine. Yes, Castrol make a 504.00/507.00 & it is probably one of the factory fills that VW uses. Others include Fuchs & Shell. My car has a Shell sticker under the bonnet somewhere
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