His logic is that it was called a sin under the old testament laws (like many things we no longer consider sins). And since Mary was under those laws at the time she gave birth, she sinned. I can’t find any evidence to refute that logic since the passage itself says a “sin offering” for “her atonement”.
Here’s a link to Baker’s Evangelical Dictionary of Biblical Theology (so not a Catholic source that he can say it’s only a Catholic interpretation).
“Other specific situations that occurred throughout the year would also require a sin offering (e.g., the cleansing of the woman after childbirth,
Lev 12:6-8 ; the cleansing of irregular unclean discharges,
Leviticus 15:15 Leviticus 15:30 ; in our age the term “sin offering” could be construed to mean that this offering focused on the problem of moral and social sin. In the Old Testament such sins were included as part of the purpose for sin offerings, but the sin offering could also be brought for physical impurities that had nothing to do with moral failure ).”
Learn what Offerings and Sacrifices means and it's Biblical definition including verses and verse references on the topic of Offerings and Sacrifices using Bakers Evangelical Dictionary of Biblical Theology Online.
www.biblestudytools.com
Another source (just a basic encyclopedia entry) also explains this
" Ritual impurity arises from physical substances and states associated with procreation and death, not in themselves sinful. Ritual impurities are in general permitted (if not unavoidable or obligatory) and in this they can be distinguished from moral impurities, which arise from prohibited acts. Both types of impurity are denoted by Hebrew terms of defilement (forms of
tame) but context and associated terms indicate that different kinds of impurity are intended."
“Many scholars have noted that the physical substances and states labeled impure, and thus deemed to be anathema to God, are associated with death and procreation. The God of the Hebrew Bible does not die and does not have sexual relations. These are characteristic of humans. To be eligible to approach the sanctuary, God’s residence among the Israelites, humans must separate from that which makes them least God-like: death and procreation. The ritual purity laws requiring separation from sources of impurity are thus essential to the frequent priestly exhortation to be like God (
imitatio dei ) and to strive for holiness.”
https://www.encyclopedia.com/religi...anscripts-and-maps/purity-and-impurity-ritual