Beeper has done a ton of open source work on matrix bridges, both themselves and through sponsoring other developers. I don't see how it's out of place to compare them to libpurple devs at all.
And by the looks of it, many of their matrix bridges were created by their, now Lead Architect, before they joined Beeper. Those are under GPL so Beeper doesn’t has much choice but to keep them open source.
So it’s kind of like me bragging about doing good for society by virtue of me paying my taxes.
Yes, the clients (both beeper mini and beeper cloud) are closed source. That's their business model - open source bridges that anyone can run if they wanted, then they just make it more convenient if you use their services by hosting it all for you and giving a nice polished client. The comparison was to libpurple devs - bridges are the equivalent of libpurple. This is like if libpurple devs decided to write a closed source client based on libpurple and then charge for it. Sounds good to me if it lets them keep working on the open source stuff.