Only the web site was down not the DPoS. Other websites accessing the same blockchain or users/services accessing the blockchain directly with a node were still up.

The Steem DPoS blockchain has never been down since some software bugs crashed it once or twice in the first few months.

Both steemit.com and busy.org were DDoS attacked. However, chainbb.com and some other web sites were usable.

The reliance on a centrally hosted web sites for UI with high costs for DDoS mitigation is indeed a vulnerability for the platform as it is envisioned (social media) but not for the blockchain itself.