I don’t believe they are officially with us yet. Coach Whitt was not at liberty to talk about them in his press conference yesterday, nor was he able to talk about any HS players that had not officially signed yet (which at the time was only one player).
Transfers can happen at any time; they don’t really have anything to do with signing day. My understanding is that there are additional processes that transfers must go through, mainly pertaining to officially enrolling and transferring academic credits from their old institution to their new one, which requires administrative oversight that takes time, especially at the time of year when schools are wrapping up exams and working to post grades from fall semester.
I don’t recall if Damuni is coming over as a grad transfer or as a 5th year senior (the 5th year being the extra year granted because of the Covid season). If he is a grad transfer, then I believe he also must deal with being officially admitted to a grad program, which is its own process with its own application, review, and acceptance windows.