There was a firmware change in 2016 that blocked 3rd party bulbs but it didn't take long for the public pressure to influence them enough to revert it.
Philips Hue hubs do work with 3rd party ZLL endpoints. However, the likely reason they disabled 3rd party device support was because all Zigbee products have compatibility issues; nothing works as well as staying with one brand.
You can't always control all features and can't update firmware of all 3rd party devices from any hub. Sometimes you just need to manually configure the drivers (in code).
According to Phillips [1] it is IKEA not implementing the standard as they expect it to be implemented. That said, it's hard to say who is right here. Either way, consumers are losing out.
As a side note, IKEA's brightest bulb is 1000lm which is much brighter than the brightest Hue bulb, and it's cheaper :) I can perhaps see why Phillips would not go out their way to resolve this issue.
The Hue Bridge connects an IP network to a ZigBee network. That component emulates the IP side: it accepts IP commands for lights and translates them to Home Assistant devices. That's handy if you have something like an Amazon Echo that knows how to communicate via Hue's IP protocol.
What I believe you are describing would require a ZigBee radio and support for the ZigBee Light Link protocol (or maybe ZigBee Home Automation?). Luckily, there's a PR being worked on to support that in Home Assistant: https://github.com/home-assistant/home-assistant/pull/6263
However Philips appear to be modifying Hue software to prevent it. Cloud lock-in. https://developers.meethue.com/comment/2337#comment-2337