That's actually not quite true.
Here are the 5 volumes the closest ever to a One Piece volume according to Oricon:
n°5 - 2009
One Piece 55: 1,810,410
Nana 21: 1,505,575
Difference: -304,835
n°4 - 2013
One Piece 72: 2,354,005
Attack on Titan 9: 1,950,807
Difference: -303,298
n°3 - 2016
One Piece 83: 2,287,521
Attack on Titan 18: 1,983,248
Difference: -204,273
n°2 - 2008
One Piece 49: 1,544,000
Nana 20: 1,431,335
Difference: -112,665
n°1 - 2008
One Piece 49: 1,544,000
Nana 19: 1,645,128
Difference: +101,128 (!)
(The other One Piece volumes of 2008 sold the following :
One Piece 50: 1,678,208
One Piece 51: 1,646,978)
While Oricon's 2008 numbers are less than reliable, it still seems like Nana did give One Piece a run for its money, meaning that pre-boost One Piece did not dominate the market that hard.
Moreover, One Piece exploded in popularity in 2010-2011, Attack on Titan came close to it after, not before, thanks to its 2013 boost. And that's not even all, since the closest AoT ever came to One Piece (Edit: I'm talking about sales per volume) was in 2016, way after both of their boosts!