The only mention of Bitwarden is in the article that you linked is an untrue claim made by a representative from Proton, a Bitwarden competitor.
The article says the following about ProtonPass:
To make matters worse, this sensitive data is not wiped from the memory when the vault is locked post-login, making it susceptible to exfiltration by info-stealer malware or attackers with physical access to the target machine.
The above is not true of Bitwarden.
However, while the vault is unlocked, of course the passwords have to exist in unencrypted form in the device memory — how else are you going to use the passwords?