Yup, I asked about that too and was given an example that made sense and the agent was fully confident in:

When you qualify for Mosaic in year 1 (lets say 2022), you have a benefit that you can pick any point between the day you reached Mosaic and the end of the following year (i.e. the end of 2023). If in year 2 (2023) you are already Mosaic (bc of your flying in 2022) and then fly a lot that year and get 50 tiles in 2023, you now have the ability to select a benefit until the end of 2024 -- you can either re-select the benefit you had and simply extend it one year, or switch to a different one.

Tiles, though, are ALWAYS applied to the year in which they're picked, never to the next year. So, when it says it's valid until '24 it means you can now select the 20 tiles at any point in 2023 or 2024, but it will only be added to the tile tracker for the year you select it in.