In short: T-Mobile has sent out its annual press release touting that the Google Pixel 3 can still be brought over to T-Mobile's network, even though it isn't selling it. It's likely that the Pixel 3 and Pixel 3 XL will be another Verizon exclusive, like the previous two generations have been. Though T-Mobile isn't confirming that, it is confirming that it won't be sold in T-Mobile stores.
T-Mobile is also touting that its speeds are faster, and is using the Google Pixel 2 and Pixel 2 XL as an example. According to data from Ookla (the developers behind Speedtest.net), the Pixel 2 had an average download speed of 38Mbps, and upload of 12Mbps. While the Pixel 2 XL had an average download speed of 37Mbps and upload of 12Mbps. Over on Verizon, the average download speed was 34Mbps and upload was 10Mbps on the Pixel 2, with the Pixel 2 XL showing average speeds of 33Mbps and 10Mbps respectively. That's not a huge difference, but that can make a big difference when browsing in congested areas.
Background: T-Mobile does usually troll Verizon with the Pixel launches these days, and in the past have offered a $300 credit for those that buy a Pixel from Verizon and bring it over to T-Mobile. So far, T-Mobile hasn't announced a promotion like that, but it will likely be coming soon. It's a way for T-Mobile to get in the news, even though Verizon is actually the one with the exclusive.
Impact: The Pixel 3 and Pixel 3 XL will be announced tomorrow during Google's hardware event. Many are expecting it to be another Verizon exclusive, as it has been the past two years. There have been rumors of the device coming to other carriers, but with T-Mobile putting out this press release, it definitely does not sound like it will be coming to T-Mobile. But you can still buy it unlocked from Google and pop your T-Mobile SIM card in. The only downside there is that you can't upgrade to it on JUMP!.