Genuinely curious.
Is it such a huge deal that the knowledge an employee is gaining at his employment is being used for personal financial gains on a different platform?
This happens all the time. This isn't insider trading. This is more akin to financial knowledge and research gleaned by an advisor in his employment and then making his own investments. Or a realtor who starts getting into investment properties.

If he's making those investments based on the "knowledge and research" he's gained in his employment, where said knowledge and research isn't available to the public ... that's pretty much the definition of "insider trading".
The knowledge is public afterwards. He also did not play on his own site so it isn't "insider" information.
A research analyst at Fidelity hears from the CFO of a company he follows on Thursday that their quarterly earnings (to be announced publicly next Monday) are gonna miss expectations. He logs onto his account at TD Ameritrade Friday morning and buys a shipload of put options on the stock.
By your yardstick, he's all good. Right?
No because that isn't the same thing. The employee only had draftkings knowledge. He didn't have fanduel knowledge.
If you don't think there's a significant correlation between FD and DK ownership percentages or that having that knowledge ahead of time isn't
massively valuable, then I'm not really sure what I'm going to say to convince you.
Personally, what I'm convinced of is that "insiders" at FD have been sharing these percentages with their DK counterparts, and vice versa, every single week (or day, in the case of MLB/NBA/NHL). Why? Because human greed knoweth no bounds.