Not sure that first bit of advice is a good blanket policy, but it depends on who's deciding how significant a negative trait must be to constitute a red flag. Some will take this advice to mean you should expect nothing short of perfection and that's just unrealistic. If you walk away from every flaw you find, you'll be alone. My partner and I have been together 13 years. We are well aware of each other's flaws, so we work on them and do what we can to mitigate the impact they have on our relationship. But we also both know that if either of us does something egregious that crosses the line, that ends the relationship. Unconditional love is stupid. There should always be conditions.
Anyway, I think it's useful to pair that advice with "know what your red flags are" so you can identify and separate the imperfections from the deal breakers.