I'm Russian, and I can read and understand about 90% of their sample texts. It reminds me Serbian (though I don't know it or any other Slavic languages, and I've read more Serbian and Macedonian than any other Slavic language, so maybe there's a bias because of this).
BTW, Bulgarian is harder to read for me than Serbian or Macedonian.
We have a lot of turkish/altaic, or just old bulgarian (before our tribe came to the Balkans) words. I'm no historian, and even they do not agree, so take my words with a grain of salt.
I have noticed that we have for a lot of things - two words, one slavanian - that russian/serbian/polish/etc would understand, and one turkish-like that arab would understand.
I wonder if it seems close to the languages one is familiar with.
My native tongue is Romanian and I grew up on the border with Serbia - I used to speak Serbian near-fluently (but lost it after 2+ decades of not using it); I also know a little bit of Russian. This language seemed similar to Serbian to me.
BTW, Bulgarian is harder to read for me than Serbian or Macedonian.