Yes - your conclusion, as well as the other answers, are correct in that the PIN code adds nothing to the security of password storage for a logged in user against certain threats, rather, it's (probably) there to counter opportunistic threats.
Here's some doco to back that up, from Microsoft Edge password manager security Aug'22 versions 77+ ...
However, physically local attacks and malware are outside the threat model and, under these conditions, encrypted data would be vulnerable. If your computer's infected with malware, an attacker can get decrypted access to the browser's storage areas.
However, using a password manager that's keyed to the user’s operating system login session also means that an attacker in that session can immediately retrieve all the user’s saved passwords. Without a password manager to steal from, an adversary would need to track keystrokes or monitor submitted passwords.
... and a bit further on, in the event of data at rest being stolen ...
Despite its inability to protect against full-trust malware, Local Data Encryption is useful in certain scenarios. For example, if an attacker finds a way to steal files from the disk without the ability to execute code or has stolen a laptop that isn’t protected with Full Disk Encryption, Local Data Encryption will make it harder for the thief to get the stored data.
A second document also talks about using the PIN code to guard against opportunistic threats, Additional privacy for your saved passwords Jul'21 (note the word privacy) ...
However, this latest update isn't a fix-all. It's very important to understand what this feature can do and what it can't. This is only a basic level of deterrence that provides an additional safeguard for your stored passwords. To best protect the passwords you’ve saved in Microsoft Edge while others are using your device, Microsoft recommends that those users sign in with their own user account on your device.
Important: This setting can't guarantee protection against malicious hackers or protect you against a motivated attacker. Malware or keyloggers installed on your device will still be able to read your passwords and attackers who can access your device can also turn off this setting if they know the device password.
PURE SPECULATION ... it may be that in the future, some additional hardening is to be added to the Edge browser pwd storage... ?
The Microsoft security team has currently rated the impact of a worm that compromises a network of Enterprise PCS (resulting in loss of all credentials in all devices’ password managers) as more severe than the (more likely but lower impact) risk of targeted phishing attacks that compromise a single user-entered credential.
This assessment is under discussion and subject to change with the addition of new security-enhancing features in Microsoft Edge.
and from the second document referenced...
"A peek into the future"
With this helpful first step, you get additional privacy for your passwords stored in Microsoft Edge. However, in certain scenarios where a device is shared among multiple people, the device password is likely known to all of them as well. In such situations, there is greater peace of mind in having password autofill guarded with a dedicated custom password that isn't shared with others. This capability is in the works, and will be brought to Microsoft Edge in the near future.