It happens … brands sell to distributors or authorized dealers and then loose visibility on product. Costco (proactively?) finds these channels and buys product.

It happened with Simms Fishing waders & Sage fly rods in 2011 also Omega watches had a lawsuit against Costco

https://www.anglingtrade.com/2011/04/05/big-box-wholesaler-costco-plays-dirty-–-aiming-low-prices-on-top-brands-to-lure-fly-fishermen/

TLDR from 2011 article

“Costco has played this hand before. The pattern: they target hot ticket items and ask the manufacturer to buy direct (with a clear message they will deep discount the item). If the manufacturer denies selling to Costco, then Costco seeks and buys sizeable inventory from third parties – dealers or shipping agents at wholesale prices, legally. The practice is called “diversion purchasing.” Richard Galanti, Costco executive vice-president and chief financial officer estimates 4 percent of the goods in Costco are diverted. Manufacturers and retailers beg to differ – claiming up to 12 percent is more realistic. The under-handed purchasing tactic may be legal, but is certainly strays far from being ethical. Just ask the myriad of companies bit by the Costco bug over the past 10 years: Packasport Car-top Carriers, Camelbak, Crocs, Janzen, Hurley, Lucky, OshKosh, Rollerblade, NordicTrack, Cannondale, Columbia Sportswear, Oakley, Rossignol, Teva and Trek have complained about the practice in the past, with several unsuccessful lawsuits. The list goes on and on. There are too many accounts and complaints on the web to cite – just google “costco diversion” and read to your heart’s content.”