It does not say that about the Sonoff ZBDongle-E. I think that you are confusing it with the no longer sold Zigbee 3.0 dongle barebone model “9888010100045” adapter from ITead that was a previous barebone dongle that also based on EFR32MG21 (though never had the “Sonoff” branding), but that board is not the same as the Sonoff ZBDongle-E. The board 9888010100045 was not recommended because that board reportingly had a poor antenna design using a poorly tuned circuit board antenna so signal reception has been reported to be extremely sensitive to interference.
Btw, you will also find that the bellows readme is not updated often, so instead check the ZHA integration docs which has been updated more recently → https://www.home-assistant.io/integrations/zha
Comparing “ZBDongle-P” vs. “ZBDongle-E” vs. barebone EFR32MG21 dongle
verses 
versus 
| Feature/Model | ZBDongle-P | ZBDongle-E | 9888010100045 |
|---|---|---|---|
| Radio SoC/MCU chip | Texas Instruments CC2652P | Silicon Labs EFR32MG21 | Silicon Labs EFR32MG21 |
| Zigbee Stack (Serial Interface Protocol API/CLI) | Z-Stack v3 (ZNP 3) | EmberZNet (EZSP v8) | EmberZNet (EZSP v8) |
| Optional Zigbee Router firmware | Yes (9dBm firmware available from Koenkk) | Yes (20dBm firmware available from ITead) | Yes (20dBm firmware available from ITead) |
| USB to UART/Serial Converter Chip | CP2102 or CP2102N | CH9102F | CH340 |
| USB EEPROM Product Description ID | SONOFF Zigbee 3.0 USB Dongle Plus | SONOFF Zigbee 3.0 USB Dongle Plus V2 (first need to run update script if from first hardware batch shipped) | None (no USB chip EEPROM) |
| Home Assistant USB Auto Disovery | Yes | Yes (but way need to run update script if from first hardware batch shipped) | Not possible |
| Flow Control | None by default (Hardware flow control optional with alternative firmware and flipped dip-switch） | Software flow control | Software flow control |
| RF Transmit Output Power | 9dBm (firmware hardcoded), Max: 20dBm | 20dBm (default) | 20dBm (default) |
| Antenna | External (rotatable and tiltable) | External (rotatable and tiltable) | Onboard circuit board antenna |
| Enclosure/case | Aluminum all-metal shell casing | Aluminum all-metal shell casing | None |
| Length | 63mm | 52mm | ? |
| Packaging | Retail-box with manual | Retail-box with manual | Anti-static bag only |
| Home Assistant ZHA | Supported | Supported | Supported |
| Zigbee2MQTT | Supported | Experimental support as still in development by zigbee-herdsman dev, see https://github.com/Koenkk/zigbee-herdsman/issues/319 | Experimental support as still in development by zigbee-herdsman dev, see https://github.com/Koenkk/zigbee-herdsman/issues/319 |
| IoBroker | Supported | Experimental support as still in development by zigbee-herdsman dev, see https://github.com/Koenkk/zigbee-herdsman/issues/319 | Experimental support as still in development by zigbee-herdsman dev, see https://github.com/Koenkk/zigbee-herdsman/issues/319 |
| OpenHAB ZigBee Binding | Not yet, see request and discussion | Supported | Supported |
| Domoticz Zigbee Plugin | Supported | Supported | Supported |
| Jeedom Zigbee Plugin | Supported | Supported | Supported |