Disclaimer: I'm not an OpenWrt developer and can't speak for the project.
First of all, this interpretation is down to the project and its developers - arguing about semantics is (IMHO) rather presumptuous.
That out of the way, the release notes of -rc1 explicitly mentioned that luci support for managing VLANs for DSA devices would be retrofitted for -rc2, in order not to delay this first -rc even further; to quote from OpenWrt 21.02.0 first release candidate:
Known issues
- DSA support is new and might not be complete or fully working
- The LuCI web interface has no support for DSA yet
This work has shown some ambiguity in the original netifd configuration syntax between L2 and L3 configurations, which had direct functional implications and limitations (particularly for DSA and wireless bridging). Given that DSA support as a whole (beyond mere software bridging of DSA members) is new to 21.02.x (and only available at all (first as external branches, then in master) for around half a year), the last chance to fix this would be now - or to live with these problems well over the life cycle of this stable release branch; respectively to abandon the 21.02.x branch in unreleased form and switch over to a hypothetical ~21.10.x. Pick your poison, those who end up having to both do the work- and to live with potential mistakes in user configuration well beyond just this release, have picked theirs[0].
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[0] and they have even worked on smoothing the migration between the different syntax styles, trying to avoid breakage of loss of comments.