What does "Secure domain space" mean technically?
Nothing specially, as this is not a technical specification.
Here is what happens.
1) HSTS is a new standard, implemented by many browsers, that prohibits them to connect over HTTP and force HTTPS. This either happens after the first connection through HTTP (the server replies with some headers dictating the browser to restart the connection over HTTPS and keep doing that for a given amount of time - of course this is vulnerable to the first connection being hijacked) OR because the website is registered into an "HSTS preloading list", which is then included in browsers code source (in that case, there is no vulnerability, as even the first connection will be over HTTPS, never over HTTP)
2) Google Registry decided to add all its newer TLDs (DEV, NEW, APP) in the HSTS preloading list. It means no standard browser will ever do an HTTP query to any hostname in those TLDs
3) Contractually, Google Registry forces registrars to display a message during registration, to warn them about the above. Of course, it would have been too simple to just explain the above, and instead they just put some vague marketing terms, and even such nonsense as "SSL certificate". Each registrar can either use the text given by the registry, or any other one, but the registry mandates the registrar to show screenshots of its checkout before Google Registry accepts it as good enough and authorizes domain registrations through this registrar.
So in short, all that means is that due to Google adding its TLDs in the preloading list, if you have a website there and want normal browsers to access it, you need to enable HTTPS (and hence have a certificate), because it will never work over HTTP.
All the above has no consequences at all for any other protocols (which shows that "secure domain space" is a claim without merit, for anyone smart enough to remember that a domain is not necessarily used for the web or not only for that; it is an unfortunate frequent wrong shortcut to think that Internet and World Wide Web are the same thing), or any HTTP client not implementing HSTS.