Unlike a laptop or desktop, closed source smartphones are quite restrictive about what you can install. So you're not as likely to install malicious software on a closed source smartphone, compared with a Windows desktop, simply because you aren't allowed to: the only things you can install are "approved".
Whereas on Maemo, you have freedom to install any old junk, and the temptation is surely there to install things you haven't compiled yourself...
We rely on the community to check things, and for the most part, it does. We also rely on distributions, in this case Maemo and Maemo-extras, to check things and often to ensure the source matches the binary. Amd, when something is found out, if you are updating regularly, there's a good chance it will be fixed quickly.
The same applies to closed source: with their app-approval processes, that provides a similar kind of checking.
But a major difference has to be on Maemo you can install anything, from anywhere, if you are stupid or if you are tricked into it. With closed source smartphones, that's harder.
It has been said that Linux is inherently more secure than Windows, by design.
But it's also been said that Windows has so many malicious programs because of user culture / knowledge / security practices, and simply because it's the more popular platform so it attracts malicious software writers, which combined with the ease of cracking it, tips the balance strongly in its favour.