Sec 13 sub section 3 is for determining the time of supply, to pay the GST on inputs on RCM basis. this section is not a restriction or time limit for availing ITC credit. ITC can be availed till September of subsequent years.
The ITC on RCM can be only claim after being paid into cash ledger Let's suppose if the provision of RCM is done & paid in FY 2018-19 , but Credit has not claim for the same , then the time limit to claim such credit is upto September 2019 or annual return filed whichever is earlier.