Especially that part in Mark was added years later.
That seems to be the standard interpretation.
But, even if it isn’t so, that passage, in a ‘whole bible’ context is talking about a situation similar to that which confronted the Apostle Paul on the island of Malta. He was accidentally bitten by a snake; he didn’t seek out a snake and dare it to bite him.
And he wasn’t harmed. AND it was a big red alert signal (a sign) to the people of Malta that something special was going on with this man, Paul.