I got the same call and told them I'd schedule it closer to 1200. I'm pretty sure they call 600 miles in advance because for every other service interval (15K, 30K, etc), 600 miles early is no big deal, so no harm getting it done in advance -- or given that people drive ~1000 miles/month on average, calling with 600 miles to go allows the dealers to schedule a few weeks out and still get the cars in before the interval drops to 0. But it seems they don't adjust that early notification for the 1200-mile service, where 600 miles early is WAY too early as a percentage IMHO.
OP, if you've already had the service done, I would still follow the break-in guide until you actually hit 1200 miles rather than ripping it up to redline just because you got your fluids changed.
'16 Cayman GT4 (delivery pics
Gone but not forgotten:
'11.75 M3 E92 Le Mans | Black Nov w/ Alum | 6MT (owned 5/2011 - 11/2015)