That wasn't the case here - at least the flaggers looked legit to me, and they had long pre-histories as normal users. The thing to understand about flagging is that people mostly flag stuff when they see it on the front page and they don't feel it's good enough to be there. Different users have opposing opinions about what content counts as 'good' on HN—so some will upvote an article while others will indignantly flag it ("WTF is this doing on HN?") once they see it. The front page is a composite of this struggle between upvotes and flags, based on different tastes. It's actually a rather intense struggle because frontpage space is so scarce (
https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...). But it's mostly not manipulation—it's just that to most people, their own taste about what's 'good' is so obvious to them that for someone else to have a different taste feels impossible—it simply has to be disingenuous!
I have a theory which I didn't post yesterday because it's speculation—but it's speculation that fits my intuition about how HN works, so maybe I'll share it in case anyone's interested.
I speculate that some users upvoted Bolt's post because they thought it would be funny to see "How Bolt does fraud detection better" in a cage match with Stripe's "Improved fraud prevention" on the front page at the same time, as briefly happened—see #5 and #6 here: https://web.archive.org/web/20180418212112/https://news.ycom.... In other words, classic internet drama.
The two titles are so similar that I wonder if Bolt had seen Stripe's post on the front page and rushed their article out to challenge it with that baity title. (If you look at https://web.archive.org/web/20180418211511/https://blog.bolt..., you'll see that the article itself had a completely different title.) That would certainly have been clever! Either that or it was just a coincidence, but either way, some users would have seen both, would have found the collision hilarious, and would have upvoted for that reason.
Since that has nothing to do with article quality, you would expect other users to then see the article on the front page, go "WTF is this doing on HN", and flag it. This is standard HN dynamics: baity title -> upvotes -> annoyed users -> flags. That's how the front page operates all day.
If this is what happened, then Bolt only made HN's front page in the first place because Stripe's article was already there—which would add considerable irony to yesterday's antics.