I see no Church Slavic, whole book is written in Croatian language, the very same Croatian literary language as it is today.
Smichiklas is amongst historians whose words we can always take for true, although he doesn't write a source for every claim he makes - it would make his books twice bigger, plus it wasn't a habbit in 19th century history books.
His sources were Croatian archives, twice stolen by Magyars during 19th century, when Magyar aristokracy was turning nations of Hungary into the Magyar nation, and twice brought back to Croatia. This fact alone proves the importance of Croatian archives which prove that Croatia had its recognized statehood from 7th century on.