Is the SEED blockcipher still secure?
Short answer: Yes, I'd still trust SEED.

The topic you linked to is from 2013 (2 years) old and for cryptography nearly never any advances will be made for a 15 year old, rarely used cipher after just 2 years.

As the best cryptanalysis only penetrated half of the rounds back then I'd highly doubt there're any advances by now, as this is only a legacy cipher in most systems and hence not a target like AES which is in widespread use.

EDITED TO ADD(1): As pointed correctly in a comment, if you plan to secure data against nations / national intelligence agencies, you can not rely on the security of SEED and therefore must consider using AES(-192/-256) or maybe even multi-encryption with ciphers with different designs (-> SEED &amp; AES &amp; Threefish/ChaCha &amp; Serpent).