Yes, because all the recreational users who snort the drug can tell if it's dimerised. I mean, yeah, definitely, they've got an NMR machine in their nose, or maybe their eyes have the world's best microscope which can view up to the angstrom. Definitely.
Also read that post carefully. "Storage in water seems to lead to polymerisation into a purple substance."
Stuff like this can definitely happen if stored in solution (especially for a long time). Once in solution, the crystal lattice breaks and the molecules are not fixed in an ordered structure anymore. They are free to move and collide with each other, and if they collide in the right orientation with the right energy, you will get a reaction. Even so, I'm doubtful as to whether the purple substance is the aldol self polymerisation product. Since bk-2-CB is basic itself, it might catalyse its own reaction (enamine type aldol catalysis) but still I think the bulky aromatic ring would slow down this reaction significantly.