Stay in school, kids.

https://en.wikipedia.org/wiki/Doug_flag

In the academic year of 1994-1995, I ended up doing graduate work in Eastern Europe studying nationalism and ethnic minorities. Though I totally love the people, cultures and landscape of Eastern Europe, I was deeply homesick for the forests of Cascadia, specifically the Willamette Valley forests I grew up around. One day in spring as I sat on a hill with my companion, I explained to her what the landscape of my home looked like. I said those vast vineyards if at my house would be vast green forests; the distant mountains of the Matras would be the snowcapped Cascades with white clouds hovering above; and above that might be the blue sky. The three colors of blue, white and green came to mind and that the pine tree in front of us would be a Douglas fir. The image stuck in my mind and spent a lot of time obsessively drawing the flag which really annoyed my soon to be wife. That period of time was crucial in regards to what was happening in Cascadia at that point.

It doesn't sound like that much of a stretch. If you got someone unfamiliar with these flags and had them pick some colors, I wouldn't be surprised if they picked the same ones.

The flag on the other wiki cites this page for the flag

https://web.archive.org/web/20200613033606/https://www.adl.org/education/references/hate-symbols/northwest-american-republic

The "Republic" and its flag are based on the so-called "Northwest Imperative," a longstanding call by some white supremacists, including Covington, for white people to move to the Pacific Northwest and establish their own country there. Covington promoted this idea on the Internet and in various novels.

Covington, ostracized by most of the white supremacist movement, had few followers, but did have a significant Internet presence that could catch the attention of would-be white supremacists.

Then from this page: https://en.wikipedia.org/wiki/Harold_Covington

He launched a website in 1996; using the pseudonym Winston Smith (taken from the novel Nineteen Eighty-Four), Covington became one of the first neo-Nazi presences on the Internet

So if anything, it sounds like this guy based his flag off of the other one because it was popular at the time, and continues to be more well known than his one.

So again:

• two flags can have the same color scheme and be ideologically opposed to each other
• It doesn't make sense to crap on symbols because a separate group is using a similar symbol. Doing so only weakens the positive symbol and strengthens the bad one