According to https://dqydj.com/net-worth-percentile-calculator-united-sta..., the ratio between the 98th and 89th percentile of wealth actually is roughly 5-6x. (89th is around 1.1 million, 98th around 5.8 million).
I noted that the wealth gap would be larger, but it's still not enough to make the parent poster's comment accurate (filling in a few points, you have 93rd percentile ~ $2 million, 96th ~ $3 million, 97th ~ $4 million).
Of course, income doesn't map that directly to wealth, even after accounting for age (if your income derives from a salary/investments from savings, you expect to move up further in wealth than in income as you age). Those 98th percentile income software engineers won't be 98th percentile in wealth when they're 60 unless they save extreme amounts of money, but they'll enter that 9.9% if they're prudent.
And I think your point about the interests of the 9% being more aligned with the 1% than the 90% is an important one. Which is part of the author's point as well, that the 9% are really benefiting from the system in a way that those at the bottom aren't.
And I myself am an example of exactly this. I plug myself in those calculators and think "how much would I have to make to be in the 1%?" rather than "this is unfair, how do we lift more people up?"
It's reasonable to define 90% of the country as "lower class"? Sorry, no, I can't buy that definition. The top of the bottom 90% are upper middle class at least by any reasonable definition.
I doubt this particular point. The 98th percentile of earners make something like $200-$300k a year, the 89th percentile make about $100k/year[0]. That's a big difference, and it will equate to an even bigger difference in wealth, but not enough that the 98th percentile will substantially outweigh the combined 89th-97th percentiles. (Arguing otherwise means assuming that the one group has 8 times as much wealth per capita as the other).
It's also true that the 98th percentile includes plenty of people in the HN crowd. That's the point: well paid professional types (or software engineers) are not being hurt by the gains that accrue to the very top of the income distribution.
So far as they/we cluster in the same industries/subsectors that are growing those billionaires, we have--to some extent--shared interests. For instance, Amazon software engineers might gain by clawing back money from Bezos and the stockholders, but they don't have a monetary interest in redirecting pay to the warehouse workers.
[0] immediate googled source: https://investormint.com/investing/how-much-does-the-top-1-m...
I always thought the 99th percentile started a bit higher, but this is roughly in line with what I've read before.