This is true. In effect this is more Tralfaldamorian, which doesn't make sense unless you've read Slaughterhouse Five. The idea that past present and future exist simultaneously just as three different positions can exist in such a way, but that does not negate the possibility of influencing the past. There are many theories of how to travel backwards in time just as we travel forward. This includes the Retarded Wave Theory, that an electromagnetic wave has two sections that travel both forward and backward in time, but the latter is smaller and is cancelled out by the stronger Retarded Wave, and that it is possible to ride the smaller wave with specialized particles that aren't affected by the Retarded wave. It also includes the anti-space theory in which physicists hypothesize that there are bubbles of space in which physical laws are different, such that i n such a bubble time could either stop or be reversed. In a blocked universe, every moment would have a position in the space-time continuum that results in the next block. By using a physical route top travel to any of these times would be like travelling to another town. The problem is that if you have an effect on this block, like killing Hitler, you change which block is created next. Your original bloch would no longer exist, and a separate series of blocks would replace it. In this sense, it would not create a paradox due to the fact that the time block you came from was merely destroyed, yet had existed upon travel. In this sense, past present and future exist simultaneously yet can be changed by travelling through time routes.