For me I disagree. I don't think apps built on top of the Fediverse/Lemmy/Kbin need to be FOSS. If that's what you want personally more power to you, but I don't it's good to speak negatively towards those who would make another choice ("Imagine using...").
I think having non open source apps built on top of open source services is totally fine. I might still choose to use an open source app if all else was equal, but I don't think an app needs to be. Ivory for Mastodon for instance is a great app but not open source. I don't think I or anyone needs to hold themselves back from using a good app for the Fediverse just because it's not also open source. And Ivory's existenance has IMO benefited the Mastodon ecosystem.
There's overhead and responsibility as well when making an app open source, or they need to be more careful about code, or it limits what libraries they can use. I don't blame devs for not making their projects FOSS depending on their context. Or maybe they start it closed and then move to open source eventually, which I think is what Artemis (app in development) is doing for now because they don't want to deal with the challenges of open sourcing yet and want to focus on development.
Let a thousand clients and approaches bloom IMO. We don't need to be dogmatic about this. It's amazing that these systems are FOSS, but I don't think the whole ecosystem needs to or should be necessarily. If you want to use FOSS clients though that's cool, and if devs build open source or FOSS clients that's great, but we don't need to judge those who choose differently or develop them.