The BCM4389 is a two-stream mobile device, intended to replace its BCM4375 that is currently used in Samsung's Galaxy S10 and Apple's iPhone 11. With Broadcom's announcement of its 6E AP-side devices last month at CES 2020, they've become the only AX chip vendor to publicly announce a full suite of Wi-Fi 6E devices.
The differences between the two devices are shown in the graphic below.
Aside from supporting the new 6 GHz band, the 4389 adds a second TX/RX chain to its Bluetooth 5 radio (making it MIMO) and supports 160 MHz bandwidth in 5 and 6 GHz.
The 4389's "tri-band" bullet point does not mean that it has separate 5 and 6 GHz band radios. If refers to a dual-band single-stream low-power radio that can be used for DFS background scanning and to maintain association using much lower power levels. The latter use seems redundant, given 802.11ax's Target Wake Time (TWT) feature that's also intended to minimize power use. So it's likely the low-power radio's primary function will be for background scanning.
So the 7 radio channels break down into 2 x 2.4 GHz, 2 x 5/6 GHz, 2 x Bluetooth and 1 2.4/5 GHz low power.
Other features not shown in the summary above are support for OFDMA UL/DL, AX MU-MIMO UL/DL, AC MU-MIMO DL and 1024 QAM modulation.
Although it's uncertain when (or whether) the FCC will approve the 6 GHz band for unlicensed use, Broadcom said it's hoping it will be approved in time for fall 2020 Wi-Fi 6E product launches in the U.S.
E.U regulatory approval of the 6 GHz band will follow later, with the earliest being "spring 2021", according to Broadcom.