It's a question of what's required for access. In this case, they would need your password and to have had some manner of device access at some point to steal the value used by 1password to verify you at one point had the secret key. Someone with a keylogger from a random untargeted malware infection could plausibly get sufficient information. It's really good 1 factor.

To be two factor there would need to be a requirement for two factors to be demonstrated at auth time. For example, if 1password encrypted the passkeys in such a way that the passkey could not ever leave the device, like via certain types of hardware backed key storage, then unlocking the vault is proof of something you know, and the usage of the signature is proof you have the chip.
The trickery comes about in the techniques available to move the passkey between encrypted hardware devices without it ever being exposed or loosing the "device you control" assurances.

For the record, I use 1password. Just not for passkeys on desktop. I prefer the Bluetooth connection to my phone, since phones currently do a much better job providing uniform targets for what's needed to provide the proper two factor for something like passkeys.