Those who mock Jesus do mean the lines they say. When they say ‘he saved others’ they mean that ‘he healed others.’
But Mark as he writes these words down, intends us to understand these words, as Lane says, in their full Christian sense…. in that if Jesus was to fulfil his mission on behalf of humanity - he cannot save himself from the sufferings appointed by God.
There’s an enormous irony in each of the lines used to mock the Lord Jesus.
In fact all the lines that are used mockingly of Jesus by those people around him while they are meant as derisive mocking put downs, they all turn out to be true.
Mark 15:31 In the same way, the chief priests with the scribes were mocking him among themselves and saying, “He saved others, but he cannot save himself! 32 Let the Messiah, the King of Israel, come down now from the cross, so that we may see and believe.” Even those who were crucified with him taunted him.