Is a single md5 hash a good way to generate an AES key?
Assuming that the password/phrase is in fact a password/phrase, and does not already contain sufficient complexity to prevent guessing (e.g. is not a cryptographically strong random string)...


  Is this true?


No. An attacker will simply guess at the password, and a fast hash function will help them to guess faster. An attacker can try a truly ridiculous number of md5 hashes per second, and will certainly break any password/passphrase that was not generated to be cryptographically strong.


  Why is something like bcypt/scrypt not required?


Slow hashing is required, and the information in the article is not good advice. Choosing MD5 because it offers the right digest size out of the box is not justifiable: Stronger 256-bit hash functions exist (e.g. SHA256), and a larger hash digest could easily be truncated down to 256 bits

A cryptographic hash function is not appropriate for use on passwords/passphrases. 


  Is AES slow enough to prevent a brute-force attack?


No, and it is not the ciphers responsibility to be slow to prevent brute-force attacks on the key. Assuming that they key is strong enough to be called a key (e.g. a uniformly random bit string), then having a slow cipher is completely non-productive. 

So the responsibility for being slow to prevent guessing attacks falls to a step before encryption takes place, namely, the key derivation step. Functions such as Bcrypt/Scrypt/Argon2 are explicitly designed for the purpose.

Alternatively

If the passphrase was generated randomly and designed to possess sufficient entropy, then using a slow key derivation process is not really necessary or beneficial. In such a case, using something like HKDF would probably be the most appropriate.

In any case

Selecting MD5 from a list because it seems fine is not a good sign...