Thanks for bringing this up. I have investigated a bit and it turns out that the new Zip compression (default) is capable of handling zip64 without any additional work.
SharpCompress is also able to do this, but for legacy reasons require explicit opt-in to enable zip64 support (in case there is an ancient tool that needs to read the zip file).
A bug introduced with the addition of the new zip library means that the flag is never actually set in SharpCompress, so any use of SharpCompress will be without zip64 support.
Ironically, setting the flag will cause the library to switch to SharpCompress, and leaving the flag alone will correctly support zip64.
I have addressed this in a new PR that deprecates the zip64 option, as all tools that work with zip files are expected to support zip64 extensions.