Sonatype's AGPLv3 licensed Nexus OSS uses ExtJS internally. ExtJS has dual-license mode: commercial and GPLv3, the later allows it to be used in open-source apps, like Nexus OSS.
Now, is it legal to use and extend Nexus OSS in a business intranet without first buying ExtJS license?
ExtJS is licensed on number of developers, so using OSS that contains ExtJS is actually ok in an intranet environment. But then, developers from the community or from a given business can contribute to Nexus OSS Github repository, which kindda makes the ExtJS license useless. There can be endless developers contributing to the Nexus OSS project (and probably Sonatype developers could too) and no ExtJS license ever be purchased by anyone.
Is this some sort of licensing loophole, or just a misunderstanding on my part?
As always you should seek professional advice on your particular situation.
However here's my personal understanding: Nexus OSS is distributed as AGPL and the ExtJS code that goes with this version is under the GPL license. In an intranet environment you should be fine.
If you were to make a commercial plugin using the ExtJS components, then yes you would need to get a commercial license.