The creator of the project suggests that the license is for edits of the emojis, rather than the projects which use the emoji. [1] As someone involved with the project, I'm not sure how this should be communicated so that this is clear
Yeah but the creator of the project can suggest whatever they want if they don't understand the license they use.
Going over CC-BY-SA for those used to software licenses only: If I make a work which uses CC-BY-SA material I have to provide attribution. If I modify the emoji to suit my own purposes, I need to release these new emoji under CC-BY-SA, and attribution to the originals must be provided, they may not be kept proprietary.
Using it in a project is redistribution, modifying it is adaption.
The license states that you have to give attribution according to CC4 and that any edits will carry the same license, the comment in your issue page suggests open sourcing the files you use to make the edits so others can easily edit your changes as well.
[1] https://github.com/hfg-gmuend/openmoji/issues/155