[Phanx]

That means it's only receiving the "self" argument, without any information about the link. You'll have to keep track of that yourself:

        local lastLink, lastText, useGameTooltip

        local function OnHyperlinkEnter(frame, link, text)
            lastLink, lastText, useGameTooltip = link, text, showLinkType[link:match("(%a+):%d+")]
            if useGameTooltip then
                GameTooltip:SetOwner(ChatFrame1Tab, "ANCHOR_TOPLEFT", 20, 20)
                GameTooltip:SetHyperlink(link)
                GameTooltip:Show()
            elseif useGameTooltip == false then
                -- Uses a special tooltip, just let the default function handle it.
                SetItemRef(link, text, "LeftButton", frame)
            end
        end
     
        local function OnHyperlinkLeave(frame)
            if useGameTooltip then
                GameTooltip:Hide()
            elseif useGameTooltip == false then
                -- Uses a special tooltip, just let the default function handle it.
                SetItemRef(lastLink, lastText, "LeftButton", frame)
            end
            lastLink, lastText, lastLinkIsNormal = nil, nil, nil
        end

Green lines are new. Yellow lines are changed. A few lines were removed, too. (On a side note, you can just do "if X then" instead of "if X == true then" if your only need to to make sure X isn't false or nil.)