Still waiting for an answer from this Deutsche Bank in Berlin...

But I'm not the slightest surprised...

The job ads could be unreal, just marketing ads....
You're 100% right, eventually you're inundated with the marketing bullcrap. People will throw nearly meaningless words and phrases at you just to gain some added leverage.
But, I wouldn't worry about it too much. You're a smart person. Try to look past that stuff. You will find what you're looking for but it won't be in the way you expect.
Just look at this statement alone:
We mean it in the conventional sense: firms doing at least 2+% ADV in any given market.
The "conventional sense"... lol, as if there is an "HFT" authority that certifies developers?!!? On top of it the qualification has to be "firms" because there's no way a person can do >2% ADV alone. And, "any given market"... which markets are we talking about? Thinly traded pink sheets?
So, with all due respect, after you pull back the curtains,
@Databento is saying that 1/2 of their engineers previously had jobs in finance. Of course, maybe they got fired from their old jobs because they weren't very good. We can't be sure.
The mathematical reality is, HFT firms "make up the low margins with incredibly high volumes"(
1). So they trade quality for quantity. It's a reasonable tradeoff but it's not necessarily something to brag about. It's not much different than when discount brokers emerged and competed on commission. Technology has changed history but that history still rhymes.
Not too long ago the big banks on wall st. used to clear transactions by exchanging physical certificates at the end of the day. Then, some smart person decided to use a bicycle to get from bank to bank faster and they became the first HFT trader! Lol
Put things in perspective. When people have nothing of substance left to offer they're just going to try and bullshit you, gaslight you, whatever it takes to get what they want... a good employee at a cut-rate price.
The reason they need the employee is because they can't actually do the job themselves. This is simply the dynamic nature of economic specialization and technological productivity gains.
I could tell you I started my career at a bank founded by Alexander Hamilton when Modula-2 on VAX/VMS was "hot technology". I could say I consulted firms up and down Manhattan from the WTC (when it still existed) all the way to mid-town. I built software for the military, and I worked for years at the foremost telecom research lab in the world... because Ma Bell's got the ill communication.
None of that amounts to a hill of beans unless I'm trying to convince you of something. Try not to fall for the superficial showboating, we all put our pants on one leg at a time. And, you seem to have a genuine curiosity... you're the exact type of person a giant vampire squid wants to latch on to. Keeping your humanity and soul is the challenge. As far as the $$ are concerned, at some point, more will always be printed. Money is literally made of linen, zinc and a little bit of copper... basically dirt. It is the world's ultimate marketing tool (with the possible exception of sex).
Keep doing the rigorous analysis true science requires and trust
your judgement. You'll have cafe society by the short-hairs before you know it