'Relatively senior' here means 'ostensibly trusted with important tasks in the past'. Both of the creators of this application (I won't say 'founders of this startup' because that's silly) are ex-WePay.
I have to be honest - that doesn't demonstrate a high level of trust at all these days. It's sad, but true. Plus, if you say they're "ex-WePay," I assume they were just everyday developers for WePay, not critical resources.
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"Head of Product" sounds like a product manager, it doesn't even sound like an engineer at all - this makes it even less surprising that these pathetic security holes existed.
Just to be clear, that means my assumptions you lambasted were generous. Not folly.