I feel like there are a few different routes you can go with a lead single:
1. an bold artistic statement with an important theme/ambitious sonic approach
2. a big rocker/energetic track to say "hey we're back, and we're still cool and not boring"
3. a pop song aimed at the mainstream market, or at least the adult contemporary demographic of it
Something like With or Without You or Numb is clearly in the #1 slot. The Fly is mostly #2 with a little bit of #1 because of the change in direction. Vertigo, Desire, and The Miracle clearly #2. They probably thought Boots had a little bit of #1 but it was really just a #2 (no poop pun intended).
Best thing is the only time in memory that they went the #3 route*, and it's baffling because there aren't any young people interested in what U2 is doing in terms of pop music. So essentially you're directing the single at, what, suburban parents? It doesn't make them look cool, it doesn't make them look particularly energetic, and it doesn't look like they have anything important or interesting to say from a lyrical or musical standpoint. It's a love song. Now that's fine, and the * exception noted above would be Sweetest Thing, which was promoting a Best Of collection that's meant to be for a more mainstream/casual fan. But for a new loosely-themed album that tackles big issues about mortality, and was delayed because they wanted to address the current political climate, it doesn't make a whole lot of sense.
The Blackout does make sense, because it's in the #2 category but also has some #1 because of its subject matter tying it to current events. In fact, its lyrics about democracy being in danger and an "extinction level event" sound just as apt now in 2021 as they did in 2017. But for some reason, they did a soft press on it instead of a hard press of making it an official lead single, and to me it lessened the impact of SOE's release.
What you see here is U2 mixing up their desire for a hit with the need to appear relevant, and they really should have realized the latter is more important. The Blackout wouldn't be out of place on KROQ and sounds modern enough to be played alongside music from U2's descendants. It's not the type of track that becomes a hit in the Vertigo sense, but it certainly would grab more attention.
So yeah, while I love Red Flag Day and it's probably my favorite on the album, and Little Things is the serious, epic track, I think they almost made the right choice, until they didn't.