I think that there was a two day delay in the image build as the tagging script failed and needed a fix.
"do not move" means that the tag has a permanent commit hash (instead of the moving HEAD of the 17.01 branch), and the tag also sets a fixed commit hash for each of the feed definitions. In practice this means when you checkout the rc1 tag later, all feeds like LuCI, packages, routing etc. are automatically set to the fixed commit in the feed, instead of HEAD.
That change was made in the tagging commit I linked. Here the changes are shown more clearly:
src-git packages https://git.lede-project.org/feed/packages.git^31d89be9e69bac261bfe7440512cb4e0f3356255
src-git luci https://git.lede-project.org/project/luci.git^472dc4b9e2ca71c114f5da70cb612c1089b8daa7
src-git routing https://git.lede-project.org/feed/routing.git^a6c7413594a0e4b42dab42bb5fa68534e39b7d0c
src-git telephony https://git.lede-project.org/feed/telephony.git^1f0fb2538ba6fc306198fe2a9a4b976d63adb304
It has always been a challenge to explain to users who want to build the exact "release CC15.05.1" etc. that it is not enough to check the main branch sources from that release date, but also the feeds need to be from the same date.
Great that Jo automated the fixing of it for LEDE.