That's crazy. If you have the skills don't under value yourself. Don't be afraid to walk away from an offer. Never tell a potential employer your current salary and never give them a number if they ask in interviews. Ask what their range is as a response and if that matches your number, proceed. Then negotiate for the max of their range. If you get to that point, they already want you, so you have the upper hand in negotiation.
I think the mention of fintech in the text makes an implication of online store of some sort, where I could see it being profitable because it's a lot more work to be able to generate listings and accept payment and shipping information.
That was never in question. Online payment portals are Fintech. You don't have to work at IBM to be in Fintech, it includes the entire process built on top of their platform as well.
I'm saying that the mention of Fintech in the First Case would IMPLY that the WebDev also deals with Fintech. If both devs have comparable skillsets then it makes sense to compare their pay rates.