I think this is because we look for properties that are foreign to us, and skip properties that we familiar with when trying to understand the language. So, for example, you didn't notice some things taken from Polish, because you already know them, but you noticed things that look Czech to you, and thingie didn't notice Czech things, but noticed Serbian ones.
After looking at the grammar and dictionary, I find it much more similar to Czech than I thought after looking at just samples. Dictionary has quite a lot of Czech words, you'd only have to apply some rules to make it look more "modern" :-) (ija -> [i]e, throw out most of trailing i's, ični -> cký, g -> h…)
I don't know. It could be Czech… but 500 years ago :-)