Did Only Judas Iscariot Understand Jesus?

In specific instances, yes; but in general, no.

There was a moment when Jesus said, “That you do, do quickly;” and in that moment, only Judas really understood Jesus. (John 13:27b)

Before that, there was a moment when Jesus said, “ [he] has lifted up his heel against me;” and in that moment, only Judas really understood Jesus. (John 13:18)

There was a moment when Jesus said, “You are clean, but not all;” and in that moment, only Judas really understood Jesus. (John 13:10b)

Just after the foot washing, only Judas really understood what Jesus did. (John 13:7; 1-15)

And just after Jesus gave him his food, and Satan entered into him the second time, only Judas understood the instruction Jesus had given them months earlier, “If anyone will come after me, let him deny himself, and take up his cross, and follow me.” (Mark 8:34; John 13:27a)

He was by that time, you see, as opposed to handing over Jesus as was Simon Peter, whose spirit of opposition had prompted Jesus to give them the instruction in the first place. (Mark 8:31-34)

This list of examples, when only Judas really understood Jesus, is far from exhaustive; but even so, in general, it is not true that Judas was the only apostle that really understood him.