Amazon does not allow one to create a permanent link to a specific seller. You can only link to the ASIN, which will determines the seller to display on-demand.

Unless the product category or brand is "gated" (and to be fair, many are), meaning it requires pre-approval to list on Amazon, anyone can hitch onto any ASIN by telling Amazon they have that product in stock (if FBA, by sending in "that product").

I don't know about that particular product but Amazon's inventory can go in and out of stock. Also they do not give themselves the buy box all the time, they do share it with third party sellers.

It changes frequently. I tried the link this morning and it is "Sold by HandAProduct and Fulfilled by Amazon."