[Kakjens]

You had to add variable and use it for billions and trillions instead of replacing it everywhere (and forgetting to change "or 0" into "or 2"). Nevertheless, below is untested code:

--Thank you for your help 
--SDPhantom
--Phanx
--Fizzlemizz
local AXP_COLOR = { r = 0.6, g = 0.2, b = 0.1 }
--Lets set up the Frame
local derArtifactBar = CreateFrame("Statusbar", "derArtifactBar",artifactFrame)
    derArtifactBar:SetPoint("CENTER", 0, 0)
    derArtifactBar:SetHeight(18)
    derArtifactBar:SetWidth(artifactFrame:GetWidth() * .968)
    derArtifactBar:SetStatusBarTexture("Interface\\AddOns\\_Deranjata\\media\\cast\\Waterline")
    derArtifactBar:SetStatusBarColor(AXP_COLOR.r, AXP_COLOR.g, AXP_COLOR.b, 0.5)
    derArtifactBar:SetBackdrop({
    bgFile =  "Interface\\AddOns\\_Deranjata\\media\\cast\\Lines",
    insets = { left = 1, right = 1, top = 1, bottom = 1 }
    })
derArtifactBar:SetBackdropColor(0.1, 0.1, 0.1)
derArtifactBar:SetBackdropBorderColor(0.6, 0.6, 0.6)
--lets clear any text then set out centerpoint
derArtifactBar.Text=derArtifactBar:CreateFontString(nil, "OVERLAY", "GameFontNormal")
derArtifactBar.Text:SetPoint("CENTER")      
 
--Registering out watched events
derArtifactBar:RegisterEvent("PLAYER_ENTERING_WORLD")
derArtifactBar:RegisterEvent("ARTIFACT_XP_UPDATE")
derArtifactBar:RegisterEvent("UNIT_INVENTORY_CHANGED")
 
--function for beautiful numbers
local function ReadableNumber(num)
    local ret
    local placeValue = ("%%.%df"):format(0) --probably can be improved into "%.0f"
    local placeValue1 = ("%%.%df"):format(2) --probably can be improved "%.2f"
    if not num then
        return "0 "
    elseif num >= 1000000000000 then
        ret = placeValue1:format(num / 1000000000000) .. " Tril" -- trillion
    elseif num >= 1000000000 then
        ret = placeValue1:format(num / 1000000000)  .. " Bil" -- billion
    elseif num >= 1000000 then
        ret = placeValue:format(num / 1000000) .. " Mil"-- million
    elseif num >= 1000 then
        ret = placeValue:format(num / 1000) .. "k" -- thousand
    else
        ret = num .. " "-- hundreds
    end
    return ret
end
 
-- now lets update the position and text on the bar so we can always have accurate info
derArtifactBar:SetScript("OnEvent", function(self, event, ...)
    if not HasArtifactEquipped() then self:Hide() return end
    self:Show()
    local itemID, altItemID, name, icon, xp, pointsSpent, quality, artifactAppearanceID, appearanceModID, itemAppearanceID, altItemAppearanceID, altOnTop, artifactTier = C_ArtifactUI.GetEquippedArtifactInfo()
    local numPoints, artifactXP, xpForNextPoint = MainMenuBar_GetNumArtifactTraitsPurchasableFromXP(pointsSpent, xp, artifactTier)
    self:SetMinMaxValues(0,xpForNextPoint)
    self:SetValue(artifactXP)
 
--Begin Stack Overflow Fix
 
local currentAXP = ReadableNumber(artifactXP)
local nextAXP = ReadableNumber(xpForNextPoint)
 
    --End Stack Overflow Fix **** also added the variable into the self.text:SetFormattedText Below
    
   self.Text:SetFormattedText("Current %s/%s    +%d Points",currentAXP, nextAXP, numPoints)
end)

Several things could be improved, for example, reducing of used variables.