As the images show, even the best Jewish Christian Scholars (Called Messianic Jews) testify that the literal (first, root) Meaning of the Hebrew word “Olam” is “Age” and ‘not forever’ as so carelessly translated in many of our English Bible translations.
Please view the two images attached showing this and the source (well respected author/scholar itself testifying to this ‘fact’).
I don’t think that the author is a Christian Universalist.
When they “add” their scholarly “opinion” of it being an “idiom” next, these translate it to mean “forever”.
I do NOT take that path and follow scholars such as Young’s Literal Translation (YLT Bible) and stick to the “literal” meaning of “age or ages” respectively (singular vs plural) as it occurs likewise in both Hebrew & Greek Original Holy Scripture as “it is written” simply because God is speaking Factually in these verses and NOT allegorically nor figuratively in any way refuting the “idiom excuse absolutely!”. Let us be wise.