I personally believe that this is outdated. Device encryption is turned on by default now, and I can’t even turn it off on my phone.
We know for sure that your encryption key is written to the Android Keystore, which is what your option would do (not requiring the master password on reboot). Ideally, to unlock the encryption key from the keystore, it requires a credential (which it does), and this is a per-app thing (which is most likely the case). However, this behavior (or any platform-based protection) is not explicitly documented by Bitwarden. Assuming that Bitwarden uses the safest implementation may be a folly. See this kind of assumption on the Windows Hello implementation thread: Does bitwarden save master password in TPM - #15 by Neuron5569 Some people on Reddit regularly advocate for not persisting the encryption key across reboots regardless of platforms.
On an unrooted Android phone, access to the /data/data directory is restricted to the app itself. As a user, you cannot access it except through an adb command from a desktop computer. Malware cannot access the encrypted vault except through Bitwarden or operating system vulnerabilities. See Storage | Bitwarden Help Center