I highly doubt that a site that operates at the scale of Twitter would be susceptible to a DDoS; or wouldn't at least of provisions in place to lessen the effect of such an attack.
I agree with you, I think the headline in this case is ill-advised, especially since the body of the report quotes a security professional who casts doubt on a DDoS as the culprit behind today's downtime. I fear it might have been chosen because it's a more provocative headline...
Twitter is pretty quick to block access by IP. I occasionally run into it even during normal usage, when I open a bunch of profile pages.