Does weather sealing really even work on this or any other pumper/extending type zoom lens? It has to let air in somewhere.
Weathersealing is one of those concepts that is really hard to measure. There are no universally accepted metrics about the extent of weathersealing. Weathersealing does not mean weatherproof.
I take any "weathersealing" with several large grains of salt.
I don't think the Sigma 24-105 is marketed for Canon users. I think that Nikon users will be more interested in this as they don't have a simular specced lens.