That's not very surprising, given it was an early tablet UI preview. However:

• it shows what the intentions were
• there is no technical reason that would limit it to Qt apps only, since Lipstick can already handle non-Qt apps (like sfdroid)
• if the idea of partner spaces will ever take off and be released and used by 'partners' (which I myself doubt), it will need to have the ability to pin Android apps. Whoever thinks that 'partners' will develop native applications with current SailfishOS market penetration, or that Jolla will do it for them with their current financial situation, is a fool
• my original comment was based on what the original plans were (which can be seen in the videos and from subsequent announcements) - if the 'partner space' ends up being something completely different, then be it. It will make sense calling people out for not understanding what partner space is once the final implementation is released - go for it then.

Sorry, I won't waste time researching up to date arguments for/against a service I don't care for (and it's off-topic here). We can continue over PM, if you feel like it.
Their attitude towards 3rd party clients is enough for me to avoid it, but if you feel like using a service that is available for free thanks to the pure altruism of its owner, then go for it!