If you listen to the audio, you do hear the speaker say "s'tsee-as", where the first 's' is distinct from the 'c'. It's subtle but there.
/side whispers: I'm much too lazy to pronounce any language perfectly so I do wind up saying cii/cias/etc. (add in my lisp and I'm especially desperate to get rid of that horrid 'sc' mix.)
Check here for notes. https://www.duolingo.com/skill/eo/Infinitive
"Both scii and koni can be translated as "to know." While scii refers to intellectual knowledge, koni refers to knowing someone or something from experience."
Mi scias la respondon. I know the answer.
Mi konas lin. I know him.
This scii/koni difference is in a bunch of other languages too as well. Spanish (saber/conocer), German (wissen/kennen), French (savoir/connaître), etc. Even Old English had these two verbs (witan/cnawan)!