Unless the benchrest competitors are confused about accuracy, then you're right about flat-based bullets being more accurate.
Strictly speaking it's not so much that the bullets themselves are more accurate, it's just easier to get a flat-based bullet to depart the bore in a more stable fashion due to (of all things
) the flat base.
A good many match bullets are boat-tailed because they offer better wind resistance & flatter trajectory which are important to match shooters. But the extreme accuracy nuts (benchrest shooters) are using flat-base bullets.