I said "Moja babcia idzie powoli" and it was counted as incorrect although "idzie" and "chodzi" means the same thing.
doulingo is rather consistent in making you translate "she walks/goes= chodzi and she is walking/going = idzie.
Is there a significant difference between wolno and powoli?
"wolno" has other meanings, but both mean "slowly". In terms of "slowly", the difference seems rather stylistic to me... frankly, I'd go with "powoli" here myself.