Disagree, I have serious doubts you could confuse the two. I can see almost no context where 'Riot (Games)' and 'Riot (Anti-Phish Company)' could be meaningfully confused.
Some people know League of Legends, most don't know Riot Games. And I double checked: Riot Games don't own a trademark for anything related to cybersecurity.
Unfortunately that doesn't seem to stop them going after companies with Riot in their name (even though Riot is also a dictionary word) :(
Way more people already know riot games and league of legends than will ever know about your security startup, so I don't see what they're upset about for you. Hand-wringing, perhaps?