I was honestly surprised to find it in the first place, because I assumed English to be at first place given the simpler grammar and the huge dataset available.

I agree with your belief, other languages have either lower density (e.g. German) or lower understandability (e.g. English)

English has a ton of homophones, way more sounds that differ slightly (long/short vowels), and major pronunciation differences across major "official" languages (think Australia/US/Canada/UK).

Italian has one official italian (two, if you count IT_ch, but difference is minor), doesn't pay much attention to stress and vowel length, and only has a few "confusable" sounds (gl/l, gn/n, double consonants, stuff you get wrong in primary school). Italian dialects would be a disaster tho :)