Custom can seize that, they can seize anything they want to, the question is if they can make it stick.
I'd say you'd have a good chance of successfully appealing a seizure. Assuming you have competent representation, anyways. CBSA seize airsoft on the ground that they are replica firearms, and past appeals at CITT were thrown out based on the items' in questions' resemblence to real firearms. Seburo doesn't even exist as a company, much less as real steel.
Probably isn't worth the gamble, though.
"The Bird of Hermes is My Name, Eating My Wings to Make Me Tame."