Also, the combinations list assumes that the attacker knows the method/word-list used to generate the password, which may not be foreknowledge the attacker has access to, especially in cases of giant many account password brute forcing attacks.
Just because your passphrase may essentially be a PIN look up into a lookup table doesn't mean the attacker knows that or has access to the same lookup table.
You can also add additional entropy via punctuation or casing.
The point to a random passphrase is to try to avoid "human" mistakes like over favoring a subset of words, and rerolling words you don't like potentially makes your collection of passphrases more susceptible to analysis or social engineering (word association) attacks.
Like I said, it's generally better to pick a word-list you are comfortable with all the possible words than to subset a word-list you aren't entirely comfortable with.
The goal of something like Diceware is to be easy for humans to memorize but also still true random (see: xkcd's battery horse comic). If you don't need to memorize it, then yes, why not entirely generate a random sequence of letters/numbers/symbols/emoji.
The entropy calculations are probably generous. The wordlist isn't as long as it looks, because some of the words are strange and people tend to re-roll if they get something like that. It's probably better to assume there's only 1000-2000 "words" people will safely combine.