The Principle of Contingent Causation: The Impossibility of Infinite Regress.

[Angrboda]

Physicists reason like this, as the above excerpt also shows: (1) the universe is constantly expanding, (2) extrapolating backward, said expansion could not have been indefinitely continued in the past (3) Therefore, the universe had a beginning. Planck time, etc, is irrelevant to this conclusion.

What did I just tell you about simply repeating things? Dr. Lincoln is using beginning in the sense of an epoch, rather than in the sense of an event; you are equivocating on the term.

Alternative models, where the average expansion of the universe throughout its history does not hold, have been proposed under the notions of emergent spacetime, eternal inflation, and cyclic models. Vilenkin and Audrey Mithani have argued that none of these models escape the implications of the theorem. In 2017, Vilenkin stated that he does not think there are any viable cosmological models that escape the scenario.

Sean M. Carroll ... added that Alan Guth, one of the co-authors of the theorem, disagrees with Vilenkin and believes that the universe had no beginning. Vilenkin argues that the Carroll-Chen model constructed by Carroll and Jennie Chen, and supported by Guth, to elude the BGV theorem’s conclusions persists to indicate a singularity in the history of the universe as it has a reversal of the arrow of time in the past.

Wikipedia || Borde-Guth-Vilenkin theorem
[emphasis mine]

Here we see that there is disagreement among the various authors of the Borde-Guth-Vilenkin theorem as to whether it implies a beginning. Maybe they should have asked you to settle the matter for them.