Way back when r/the_donald just started when his campaign began, many people including me thought it was an ironic subreddit and treated it as such. Then suddenly Trump gained momentum and the subreddit turned out to be for real.

I'm still convinced that it was a joke in the beginning, and then the subreddit saw an influx of people who didn't get the joke.

That's definitely also my impression. The only point against that theory is the moderation. I don't think someone who started that as a joke would let it become what it was at the end. unless maybe someone related to the Trump campaign bought those accounts of course.

Irony has always been a tool of the far right, from the nazis to the Christchurch shooter. How 4chan and t_d use it is not especially different.

I agree on that, but I think this case was a bit different. A lot of the early posts were making fun of Trump pretty thinly veiled.

There were some subreddits that were taken over by the people they made fun of, /r/gamersriseup started like that before it took a turn IIRC

Yep.

Consistent irony and in-jokes become real over time.

"Ha ha only serious."