It's about intersection of "anti-competitive" and "big enough to alter market dynamics".

In your hypotheticals, if Ford controlled all dealerships then refusing to sell GM would be anti-competitive.

If Apple controlled all physical stores where computers are sold, it would be anti-competitive to refuse to sell Microsoft.

Amazon directly competes with Chromecast and is dominant on-line retailer in US so they do have have the power to alter dynamics of Chromecast vs. Fire TV competition. That's what makes that anti-competitive.

Notice that a big part of amazon.com value is being a platform to others to sell their stuff. I certainly bought stuff via amazon.com that was not sold by Amazon, merely distributed by them.

To go against that platform "neutratlity" (if you will) and refuse even 3rd parties to sell a very specific product, while allowing thousands more, is pretty damning evidence that it's straight up, anti-competitive leverage of their position in one area to gain advantage in another area while negatively affecting customers.