I am not surprised. Polish guy struggled to understand even the word "hrad" that means "castle", he also thought that in Czech 30 was 300. I am not surprised since I thought the same. I think if he would be exposed a bit more into Czech it would help. It is also easier to understand written language than spoken.
Slovak and Czech languages are similar, in the past only children stories were translated. But when I was with my mother in Bratislava and we needed to explain something to our Slovak friend in the derail, my mother had to start talking in German, since she and her friend understood it well.