It calculates 18 quintillion years because howsecureismypassword.net is simplistic and doesn't realize english words are being used. It categorizes each character into some categories (see https://howsecureismypassword.net/assets/js/app.min.js):

  // Standard ASCII Characters
  add('ASCII Lowercase', /[a-z]/, 26);
  add('ASCII Uppercase', /[A-Z]/, 26);
  add('ASCII Numbers', /\d/, 10);
  add('ASCII Top Row Symbols', /[!@£#\$%\^&\*\(\)\-_=\+]/, 15);
  add('ASCII Other Symbols', /[\?\/\.>\,<`~\\|"';:\]\}\[\{\s]/, 19);
  [...]

It sees the password is 22 characters long, and contains a-z chars (lowercase) and a space char (other symbols) so it estimates each char is drawn from a set of 26+19 = 45 letters. And it assumes the default bruteforcing speed is 4 billion passwords per second. So:

  45**22/4e9/3600/24/365.25 = ~18 quintillion years.