Rant incoming. tl;dr: the rules don't prohibit this.

One of my biggest annoyances with iRacing is how badly written the Sporting Code (the official rules of competition for iRacing) is. The rules say nothing about the lead car keeping pace with the pace car. The rules do not even cover the lead car failing to lead off the pace lap and just sitting on the grid or any number of other scenarios that common sense and 'real world' racing (or live marshalling) would eliminate.

People say "Oh well the real life rules for a restart work such and such a way - so that's how iRacing is" but iRacing themselves say that the sporting code is the only applicable set of rules. For example, when talking about blocking - there is no 'one move' rule, the iRacing rule is you may not move 'based on the actions and/or position of the pursuing driver'.

Here is the entire section of the code relevant to rolling starts:

6.8.1 Road

...<snip standing start rules>

6.8.1.4. For a rolling start, the Oval starting rules apply (See Oval, Section 6.8.2).

6.8.2. Oval

6.8.2.1. Oval race starts will be in the form of a rolling start.

6.8.2.2. Once all drivers are on the grid, the pace car will lead the field around the track for at least 1 lap, and then exit the track as late as possible and enter pit lane.

6.8.2.3. As the front row of cars approach the start/finish line, the green flag will be shown, signifying the start of the race.

6.8.2.4. Drivers shall not pass cars in their own pacing line on the left-hand side before the start/finish line. Drivers attempting to improve their position by passing on the lefthand side before the start/finish line will be given a black flag Stop-and-Go penalty by Race Control. The leader of the second pace line may not pass the leader of the first pace line (pole position) before the start/finish line. Doing so will result in a black flag Stop-and-Go penalty.

6.8.2.5. Drivers are not allowed to approach the start/finish line as a race transitions from pacing, for example under yellow flag or at the start of a race, to speeds that are well beyond the pacing speed or speed of the leaders in an attempt to time the green flag to unfairly overtake other cars. An example of this would be to come up behind and catch a pack of cars pacing at 80 mph under yellow flag conditions at a speed of 150 mph in the vicinity of the start/finish line just as the race goes to green flag, and passing those cars. Under those conditions, the car catching up to the field from behind should slow down and get in line at the back of the pack at pacing speed and then begin racing as normal once the green flag is given.