Beeper has been around since 2020 and its primary product (now called beeper cloud) is an all-in-one chat application which supports many protocols from the same client, not just imessage. Even if imessage/beeper mini is the product generating discussion right now, beeper cloud has been the primary focus of their company since the start and beeper mini is just a recent tangent. They use matrix bridges as the backend and some of the devs now working for beeper were already working on the open source bridges which beeper now uses even before beeper the company existed. Beeper was the first organization (and one of the few it seems) to become a corporate sponsor of the matrix foundation, and they also routinely sponsor developers to work on the open source bridges that make up much of their infrastructure, even beyond the ones they hired on as staff.

I'm not sure how all this points to "get-rich-quick" silicon valley startup type to you but the behavior looks pretty different to me. Given their history, my guess is they put beeper mini out there primarily to put pressure on apple. If apple doesn't shut it down, cool, everyone has access to imessage through beeper. If apple does shut it down, at least it leaves little doubt about the situation: interoperability is not impossibly hard, apple really doesn't care about the security or privacy of their customers as much as they like to say they do, and at the end of the day imessage is only limited to apple devices because apple is apple. (For most companies I would say imessage is locked to apple devices because it's about the money but apple is one of those companies that takes weirdly principled positions even when it makes no sense, kind of similar to nintendo in that way.)

I suppose if all I had seen was the past month of buzz around them then I might perceive them differently.