yeah that is the "consensus" in biblical scholarship, but it's extremely disputable, because the evidence they have is based on nothing concrete whatsoever and they all ignore the very damning evidence of Josephus, on the very slight possibility of a common source.
basically the argument for Mark being pre-70 is that Mark 13 (which has Jesus directly predict the physical destruction of the temple!) doesn't describe the exact circumstances of the Roman-Jewish war in as close detail as the other gospels, which they take to imply that it was just a wildly successful prediction made by Jesus and written and published by the author of Mark prior to 70 CE. ...which isn't even a logically valid inference anyway, a book can not refer to something or exclude written material that was available, and that's not proof it was written before that time period, just that the author didn't include that material. But more saliently, this dating entirely relies on "the Gospel of Mark successfully predicted a major historical event that happened a few years afterwards" and that's an extremely dubious assumption to make.
But yeah, anyway, here's mark 6:
and here's the parallels in Josephus' Antiquities Book 18:
Now some of the Jews thought that the destruction of Herod’s army came from God: and that very justly, as a punishment of what he did against John, that was called the baptist. For Herod slew [John]; who was a good man, and commanded the Jews to exercise virtue; both as to righteousness towards one another, and piety towards God; and so to come to baptism. For that the washing [with water] would be acceptable to him, if they made use of it, not in order to the putting away, [or the remission] of some sins [only,] but for the purification of the body: supposing still that the soul was thoroughly purified beforehand by righteousness. Now when [many] others came in crouds about him; for they were very greatly moved [or pleased] by hearing his words; Herod, who feared lest the great influence John had over the people might put it into his power and inclination to raise rebellion: (for they seemed ready to do any thing he should advise:) thought it best, by putting him to death, to prevent any mischief he might cause; and not bring himself into difficulties by sparing a man who might make him repent of it when it would be too late. Accordingly he was sent a prisoner, out of Herod’s suspicious temper, to Macherus; the castle I before mentioned; and was there put to death. Now the Jews had an opinion, that the destruction of this army was sent as a punishment upon Herod; and a mark of God’s displeasure to him.
But Herodias, their sister, was married to Herod [Philip], the son of Herod the Great; who was born of Mariamne, the daughter of Simon the High Priest; who had a daughter Salome. After whose birth Herodias took upon her to confound the laws of our countrey, and divorced her self from her husband, while he was alive, and was married to Herod [Antipas], her husband’s brother by the father’s side.
Book 18.8.7: (in an unrelated story, but the same literary motif)
But King Agrippa, who now lived at Rome, was more and more in the favour of Caius. And when he had once made him a supper, and was careful to exceed all others, both in expences, and in such preparations as might contribute most to his pleasure... .
...Caius, when he had drank wine plentifully, and was merrier than ordinary, said thus, during the feast, when Agrippa had drunk to him: “I knew before now, how great a respect thou hast had for me; and how great kindness thou hast shewed me, though with those hazards to thy self, which thou underwentest under Tiberius on that account. Nor hast thou omitted any thing to shew thy good will towards us, even beyond thy ability..... Every thing that may contribute to thy happiness shall be at thy service: and that chearfully, and so far as my ability will reach.” And this was what Caius said to Agrippa: thinking be would ask for some large country; or the revenues of certain cities
I think it's hard to argue the author of Mark 6 didn't have a copy of Josephus Book 18, given that he treated all the same topics in the same order, including seemingly adapting a story from a different chapter; it's just too much of a coincidence. So there's no way Mark could have been written much before 100 given that Josephus wrote his work in 94 CE, and once we've established that Mark can't be dated to the approximate time of the Roman-Jewish war, there's no real reason to object to it having been written a good while after 100, either.