No, that is not true at all. we have no idea as to the original content specifically in Acts. It is very problematic to claim “unchanging” content when there is no original and even the dates of authorship are in question.
There is a minimum 200 year gap in our textual evidence. Our scraps of manuscript date from a time when interpolations, editing and even wholesale forgery of texts was rife. 2 Peter, Titus, 2 Timothy are examples of this practice.
Your counter seems to be missing my specific point. We were talking about Acts not the entire New Testament. I can, if you wish take you down the path of evidence that shows that the Gospels were NOT eye witness accounts as has been accepted by every serious contemporary scholar.
As it is, even much later copies show marked differences in phrasing and content, all you have to do is look at the Codex Siniaticus texts versus the Codex Vaticanus.
If that can change so markedly in so few years then your claim that Acts is an unchanged historical document is just patently problematic.
As to your claim that “Paul” (whoever he was) spoke to witnesses of the physical resurrection can be more accurately put as: “An anonymous fan fiction writer, known to claim inspiration from his dreams, CLAIMS to have spoken with witnesses who CLAIM to have witnessed a man coming back from the dead. No evidence to back up these claims has been presented”
For those not familiar with the verse it is 15 Corinthians: Paul; admits he is inspired by a visit from a very dead jesus. He does not identify the 500 witnesses…or take eye witness testimony from any of the apostles. It is all hearsay and dream talk. " 15 Now I make known to you, brothers, the gospel which I proclaimed to you, which you have also received, in which you also stand, 2 by which you are also being saved, if you hold fast to the message I proclaimed to you, unless you believed to no purpose. 3 For I passed on to you as of first importance [a] what I also received, that Christ died for our sins according to the scriptures, 4 and that he was buried, and that he was raised up on the third day according to the scriptures, 5 and that he appeared to Cephas, then to the twelve, 6 then he appeared to more than five hundred brothers at once, the majority of whom remain until now, but some have fallen asleep. 7 Then he appeared to James, then to all the apostles, 8 and last of all, as it were to one born at the wrong time, he appeared also to me. 9 For I am the least of the apostles, not worthy to be called an apostle, because I persecuted the church of God. 10 But by the grace of God I am what I am, and his grace to me has not been in vain, but I labored even more than all of them, and not I, but the grace of God with me. 11 Therefore whether I or those, in this way we preached, and in this way you believed.
Hardly reliable, and certainly not something to rely on for your life.