Not necessarily true. A fintech company will give you rich understanding of how the business operates and how the law interacts with it, but in a the area of business the company work in. At a top tier focussed fintech practice, you will be exposed to a variety of fintech businesses - payments, wealthtech, lending, neobanking - just to name a few broad categories. AZB, Ikigai Law, Spice Route Legal do a lot of fintech work.

If you really want to do fintech work then join a fintech company. Most of the interesting fintech work doesn't go to firms since that requires quite a bit of background and understanding of the business, which is varied and specific and its hard to brief someone from the outside about it.