Can Steem be used as smart contracts platform? If No why(in the light of the article below)?

I think that Smart Contracts are possible on Steem, as Dan states in the article, but the process to add one is complex. Adding new Smart Contracts isn't a part of the day-to-day functionality of the Steem blockchain.

This is a really good summary:
https://steemit.com/steem/@therealwolf/does-steem-need-smart-contracts

It's worth reading the whole article but one key phrase to note is:
"So while STEEM currently has the ability for smart contracts - they have to be written in c++ in addition to being added to the Steem-repository, which is not good enough for app developers."

I think that the development of SMTs will bring some smart contract functions to the Steem blockchain, but only those that the Steem developers code specifically to handle SMT functionality (toke issuance, liquidity etc). You can read the SMT whitepaper here:
https://smt.steem.io/smt-whitepaper.pdf

I would add that there is a good argument that this approach fits well with Steem's niche, which is bringing monetary rewards to content production (whether it's blogging, vlogging, game streaming, open source coding, tweeting, product reviews or Q&A). There are now many blockchains out there with smart contract functionality but none are really competitive in the content production space and so focussing to keep Steem the leader in this area is a logical choice.