It should also be noted that on about 400 short texts (~300 words each), it did not correlate with the Flesh-Kinaid readability measure at all. So it's not measuring something like average word length or syllable counts.

But it's not QUITE a true lexicon, as it handles Out-Of-Vocabulary words quite strangely. If you use as input text:

"PR-Experts, politicians, ad writers or scientists need to be strong here! BlaBlaMeter unmasks without mercy how much bullshit hides in any text. A useful tool for everyone involved in writing! Simply copy your text into the white field and check your writing style. It works with english text up to 15.000 characters (overhead will be cut off). For a meaningful result we recommend a minimum length of 5 sentences."

Then you get 0.16. If you replace the last word 'sentences' with 'strategy' you go up to 0.44. However, if you change the last word to 'sentstrategyences' you get 0.47. Try it: you can basically insert 'strategy' inside ANY word and really raise your score. Actually, if you just insert "strateg" anywhere inside the text, it goes up massively.

So I actually think it's just doing string search counts over a lexicon.