The hybrid-efi tag does not currently exist. (I don't just mean that no questions are currently tagged with it. It doesn't exist at all; no question has recently been tagged with it, and it probably never has existed. You can verify its nonexistence by typing it in heretyping it in here.)
Tags are only synonymized once they exist. Tags are created by a question being tagged with it (either originally, or in an edit). A tag is never created without a question.
Users with 300 or higher reputation can create new tags. There are many such users, but this tag has never been created (at least not yet). That suggests we may not need it.
Furthermore, you say:
So from a user point of view hybrid-efi is very different from efi/uefi capable machines that do use shell prompt as well as having gui.
It sounds like you're saying hybrid-efi has a substantially different meaning from uefi or efi. If that's the case, it shouldn't be a synonym at all.
To synthesize these two arguments: The tag doesn't exist, so we don't know for sure how people would use it. If it is created and turns out to express the same thing as uefi, then it should be made a synonym. If it's created and turns out to express something different, then it should be kept as a separate tag. Either way, new tags shouldn't be created artificially to be made synonyms--instead, we should wait for it to be created. If it ever is, we can see how it is being used here and act accordingly.
