Entropy checkers are based on a faulty premise and are always at a disadvantage compared to the state of the art in password cracking. Online security would be in a better place if they had never been invented at all.

The only way to guarantee the entropy of a passphrase is to generate it from a source of high quality randomness (e.g. a CSPRNG or by actually rolling dice, hence the name "Diceware"). Diceware-style passphrases simply balance this requirement with usability by optimizing for things humans are typically good at memorizing (strings of words).