Some people are disputing it, fwiw. But one thing is clear: the Passover lamb was not killed to atone for anyone's sins. The Jews use a goat for that purpose and that happens on Yom Kippur, which is nowhere near the Passover.
Again, animal sin offerings were for involuntary sins only, which is a big fly in the Christian ointment.
As for the Tanakh, the book of Isaiah which is found in the oldest known Tanakh (9th century AD, Moscow), and all subsequent copies of it, agree fully with the Qumran version of Isaiah.