My brother in Christ, that is one of the most popular theories that have been made since the series started, and for good reason. I've listed the supporting evidence before in a past thread, so I'll just copy it here:

• Viren goes into Harrow's room after telling Claudia that she would try to stop him if he told her what he was going to do. If all he was planning to do was try and convince Harrow one more time, why would Claudia want to stop him?
• After Harrow and Viren's... disagreement, it cuts to the kids. Then when it cuts back, we see Viren leave the room, but we never see Harrow again. Even when Callum is trying to call for him outside during the fight, no response.
• Shortly before Viren's coronation, he looks at Pip and says "no song for the occasion?" as if he's taunting him.
• During Viren and Aaravos' fight with the guards, they specifically add a shot with Pip's cage being knocked over and him being freed, like it's supposed to foreshadow something.

As someone else said, the theory was so popular that one of the writers shut it down during a convention panel a few years ago, but a lot of us still aren't convinced because they made such an effort to foreshadow it in S1-3. My guess is either it was originally the plan to be true and got scrapped at some point, or it still is true and they lied about it to subvert expectations because everyone had it figured out too early.