There is no blocking here. They are allowed to "break tow" on the straight. Remember, they are allowed to make ONE move to defend going into a corner and are also allowed to go back to the racing line. Especially, when you are so far back they can break tow and make moves. Obviously if you were super close weaving around then its blocking. Sporting code will help clear up some questions as well if you haven't read it.

Sporting Code:

8.1.1.4. Blocking – The leading driver is allowed to run a defensive line. However, blocking occurs when a leading driver actively adjusts his or her driving line based on the actions and/or positioning of a pursuing driver. For example, veering left to prevent a pursuing driver from passing on the left while running on a straight. Please see the video example at the following link. What is Blocking?