What you just described is fine but different to what the guy in the article described and his conclusion wasn't further investigation it was him having his mind changed. He had three things going on in order to prove why his mind would be changed: his memory of doing the sum previously, his observation and what he called mental confirmation, which was doing the sum in the present.
I believe that what he describes is impossible because he tried to separate two things that can't be separated.
It was a fun mental experiment but fundamentally flawed because his assumption of how memory works seems to be wrong.
There aren't many reliable things in the universe but pure maths is one of them-thank God!