Why are some contrib module functions not recognized?
Do module load include of twitter.inc in which above function was defined.. .module files are automatically included, but .inc are not. So you need to include them using above function...

module_load_include('inc', 'twitter', 'twitter');
dpm(module_load_all()); // returns 1
dpm(module_exists('twitter')); // returns 1
dpm(twitter_account_load('@twitter'));