You are correct. He laid down on that cross voluntarily. He could have prayed to the Father and had 12 Legions of angels there to fight for Him. I am the one who drove those spikes into His body.
However, there may be a second aspect to this passage.
If John chapter 5 is taken literally, all will be resurrected at the same time, upon His return.
John 5:28 Marvel not at this: for the hour is coming, in the which all that are in the graves shall hear his voice,
John 5:29 And shall come forth; they that have done good, unto the resurrection of life; and they that have done evil, unto the resurrection of damnation.
We see the same idea in the Book of Daniel.
Dan 12:1 And at that time shall Michael stand up, the great prince which standeth for the children of thy people: and there shall be a time of trouble, such as never was since there was a nation even to that same time: and at that time thy people shall be delivered, every one that shall be found written in the book. (Book of Life?)
Dan 12:2 And many of them that sleep in the dust of the earth shall awake, some to everlasting life, and some to shame and everlasting contempt.