But didn't understand how that applied to my question. What should I do for development in Drupal?
Symfony 2, as in the full stack Symfony 2 has no bearing on Drupal 8 -- we do not use its bundles, we do not use the same routing as Symfony does (we use a botched up work on top called Symfony CMF which is made less dog slow by bastardizing my menu work on top of it but it's
Edit: we do use the service container http://symfony.com/doc/current/book/service_container.html at least the static parts of it -- however dynamic service additions and alterations are done via Drupal specific means (service providers which attaches to the system via a special magic way which is neither events nor hooks. Welcome to Drupal 8.)
So: if you know Symfony 2 inside and out then a little of that will be usable in Drupal 8. If you don't then don't bother with Symfony 2 books or tutorials.
I "grew up" on Drupal so I was always procedural guy(Drupal is my main/only tool for making websites which is my job). At first I was very confused when I started to use Drupal 8. The OOP wasn't that hard since I knew OOP, but the new architecture was hard to get my head around. But after two weeks I started to feel like home again. This website and #drupal-contribute helped me a lot. I've read some information from Symfony documentation to understand controllers basically and no I'm working with D8 for 4 months and I feel like home again.
Things have changed but you'll learn the new ways very fast. You really don' have to learn Symfony AT ALL. I didn't.
One thing that made things MUCH easier for me was switching my IDE from NetBeans to PHPStorm because NetBeans didn't supported browsing inherited methods. After few months I cannot understand how was I able to use NB :) (though working with Git in NB is for me much better)
So to answer your question: you absolutely don't have to learn Symfony. To meake your life easier look at *.services.yml files in modules(that implements them) and also route callbacks in any modules *.routing.yml file to understand page callbacks(from hook_menu in D7).