I'm not sure how this round of packages got seized. There is always the possibility that a drug dog detected it. After all, seeds originate inside a bud so they do have THC molecules all over them which can be smelled by a drug dog. Typically, darknet vendors will seal the packages up with multiple seals and usually put a moisture barrier bag (think Capri Sun packaging) sealed around the outside. Then they clean off that outer seal with rubbing alcohol to get rid of any remaining THC molecules. That way the dogs won't be able to smell it as long as the package isn't sitting in the mail too long. Smells will eventually get through any type of barrier, but depending on what type of barrier it is it takes a certain amount of time for those smells to permeate the packaging.
Personally, I don't believe in this case that it was smell that got the packages busted. Take this with a grain of salt, but my assumption, being that Gu has many enemies and competitors in the seed business which I know about from reading the Green Point seeds thread on this site, my assumption is that he might have been ratted out by one of his competitors because one in particular, who absolutely hates him, lives in the same area and knows all about his business.
Keep in mind that if they discover one package with seeds in it, they will easily be able to round up all of the other packages because they have similar packaging and usually the same return address, Etc. If he wasn't ratted out by a competitor, then I'm not really sure how they discovered it.
I am just glad that they don't see it as a very big deal or take it very seriously for seeds. I think if this happened 20 years ago, I could probably have expected a SWAT team to kick down my door with a search warrant, but maybe I'm wrong.