As the CLA states the copyright isn’t touched by the licensing [1]. Every contributor will keep her/his copyright by contributing to the project. By signing the CLA every contributor grants the TW project a very “relaxed” license - forever.
See: 2.1.b of the CLA
(b) To the maximum extent permitted by the relevant law, You grant to Us a perpetual, worldwide, non-exclusive, transferable, royalty-free, irrevocable license under the Copyright covering the Contribution, with the right to sublicense such rights through multiple tiers of sublicensees, to reproduce, modify, display, perform and distribute the Contribution as part of the Material; provided that this license is conditioned upon compliance with Section 2.3.
PS: I’m not a lawyer, so everything mentioned here is my interpretation of how things work and therefore it may be wrong.
[1] TiddlyWiki5/licenses/cla-individual.md at master · TiddlyWiki/TiddlyWiki5 · GitHub