You're misunderstanding the bit of that blog post that you've quoted - it only says that starting to analyse t.co links for malicious content is a good thing for reducing malicious content. It says nothing about whether that job is easier than if they started doing the analysing without having a link shortening system in place. And in fact it goes to show that spam prevention wasn't a motivating factor in their creating t.co shortener in the first place, since they only started analysing for malicious links later than they started shortening links.

It is possible that their task was made easier by having the t.co system in place - impossible to know without understanding engineering details of how Twitter and t.co work - but it's not a claim they've made publicly, nor a reason for their deciding to use shortened links.