This appears to be a Electron custom page implemented in multiUserUi.nsh and the text is defined in assistedMessages.yml. It looks like nsisLang.ts is used to convert the .yml file to a temporary .nsh include file.
It might be possible to use the Electron NSIS include feature to include your own custom file. Depending on when it is included it might be possible to override the string:
; NSIS include file:
!pragma warning push
!pragma warning disable 6030 ; Disable multiple use warning
LangString onlyForMe 1033 "Just me and only me" ; 1033 is English
!pragma warning pop
If that does not work then you must manually edit assistedMessages.yml instead.