Yes it is a widely accepted implementation technique. pbkdf2 is a key expansion algorithm (key derivation function) that is used to create a key that is more resilient to traditional bruteforce attacks. It works by repeatedly computing an MAC or hash function against the initial provided key to derive a longer, more resilient key. It's also a slow-by-design algorithm, which increases the computational resources necessary for an attacker to attempt to brute force the whole key space thus limiting the impact of encrypted data being stolen.

It takes a key of lesser length and expands it. This increases the duration an attacker would need to break the key. This key is then used for AES encryption. So yes they are using AES-256 with pbkdf2 for key expansion which is fine. I'd be more considered about their claims of zero knowledge and where the proof on that is. But does this help answer your question?