As a developer from that era - developing for Symbian was absurdly difficult and awkward.
Around 2007 we tried building an app that scanned barcodes in a store to show you comparison from online stores. Symbian's APIs for cameras were so broken, that we couldn't get a decent enough resolution of photos. Java had permission/ux issues that made the app unfeasible. It was literally impossible to build an app like this back then, even though phones had good enough cameras already.
Not to mention the fact that you needed a whole studio to build and test even a simple app like this - so many variants of OSes, and phones, and every single one had it's own quirks.
And to add an insult to the injury - we had to pirate the SDK, because the official links to SDKs didn't work for a few days, and nobody seemed to notice.
So yeah, in theory the users could download apps to the devices. But in practice it was easier for devs to hack into iPhone 1.0 and build apps there - even before the official app store launch, than it was ever to build something for Symbian and the others.