That's not about putting their eggs into separate baskets, it's about Nokia's shortcomings in terms of both management and engineering.
I worked very closely with Nokia during that time period. As closely as one could work without being directly employed (we deployed a product directly to the S60 baseline).
What happened at Nokia was the end result of years of engineering neglect. Symbian was a monster. It was poorly engineered in general, and certain subsystems (like messaging) had a reputation for simply being impossible to work with. It was just a huge mess.
The UI saga was just another symptom of that mess. That touchscreen phone referenced in the article? It existed well before the iPhone in various incarnations. They spent years trying to push that thing over the finish line, and failed. The underlying graphical subsystems gave them problems. The new form factors presented by the emergence of touchscreen slate devices proved at odds with a whole host of assumptions made throughout the (huge) Symbian/S60 codebase.
The whole thing was just a giant hot mess.
It's possible that competing internal frameworks could have worked. The ultimate problem is that they never developed one compelling UI paradigm/library that actually worked. Much less worked in a well integrated way across the whole platform.
Nokia failed at project management. They failed at engineering. They failed at leadership.
Now they're just another OEM.