I think that both you and the grandparent comment are correct at the same time. It feels like there is a strong dichotomy between how H1B visa is treated across different realms of tech.
I have been working in big tech for a decade, and have a few friends working in the more technical financial firms (think more TwoSigma/HRT/Citadel, not JPM/GS), and my experience matches yours. Every H1B I knew at the companies I worked at was absolutely not hired as a cost-cutting measure. It was just simply difficult to find people who could qualify. And god knows we tried, I remember my team interviewing roughly 50 people just to fill a position of just a baseline decent dev (not a rockstar math olympics winner with multiple groundbreaking publications; though that would've been nice). And absolutely none of them were underpaid either, they were on the exact same comp range for the level, with plenty of them topping out the range. In fact, they actually ended up costing more to the employer than a local would, due to having to deal with immigration lawyers/filing fees/etc. (all while being paid the same as everyone else).
Meanwhile, I have a lot of acquaintances from undergrad working as devs at tech consultancy firms and more "general/traditional finance" companies. The stories I hear from them about H1B at their workplaces are pretty much exactly what the grandparent comment describes, and even worse.