Well, no. From observations so far (2013-now) basically every time a community initiative hit any UI work in the closed source components only Jolla developers can do things ground to a halt and have not moved since. In some cases developers switched to alternative solutions (all the UI patches, various IM clients).

Just from top of my head:

• one category in the latest coding competition was won by patch adding GPG to the email client
• SIP is still waiting for UI integration (5 years and going strong!)
• third party applications still can't put settings pages to settings
• no multituser chat support (also quested by many users from day one)
• someone even tried to submit patches to the closed calendar UI "upstream" via the mailing list, only to be ignored
• usb tethering is still more or less hidden (by not being installed by default) for years due to missing UI elements (as discussed on a community meeting a while ago)
• AFAIK no new accounts or messaging providers have been added to the build in messaging system by a thirparty since Sailfish OS launched

So given this track-record it seems to me like a totally sensible idea to develop all these UI elements as the effert will not be blocked for years by requiring overworked Jolla developers to participate in it. I'd like to be proven wrong (and everyone is certainly welcome to chime in with any counter examples!) but it really seems to me like the only way to go.