The Big Bang is the beginning of the current expansion, not necessarily of everything. Inflation, bounce models, and quantum gravity approaches all allow a past before it. Isn’t “the universe had a beginning” just one reading?

That is accurate and worth saying plainly. The standard model traces the observable universe back to a hot, dense state about 13.8 billion years ago; what happened at or before the Planck time is not known, and several models extend the past. What can be said with confidence is that the observable universe has a finite age and that eternal-past models face the Borde-Guth-Vilenkin theorem’s constraint on average expansion. The careful version of the claim is that the observable universe has a finite past and that models with an infinite past face serious theoretical obstacles, not that physics has closed the question.