Slack API - sharedPublicURL fails with UserToken of user that is not in the channel

Question

I created a Slack app with an admin user (let's call it Admin) and installed it to a workspace (let's call it wspA).

From the app's Messages tab, I'm trying to send a message with an attachment with a different, regular user (let's call it User) and share it to a different workspace (wspB).

When the message event reaches my backend and I call sharedPublicURL with Admin's xoxp token I get a file_not_found error. I did set the files:write user scope.

The code looks something like:

const { WebClient } = require('@slack/web-api');

let client;

const init = () => {
    client = new WebClient("<bot's xoxb token>");
};

const forwardToOtherWorkspace = async ({event}) => {
    const attachments = await Promise.all(
        (event.files || []).map(async file => ({
            ...
            url: await getPublicUrl({ file })
        }))
    );

    // send the attachments to wspB
};

const getPublicUrl = async ({file}) => {
    if (file.public_url_shared) {
        return file.permalink_public;
    }

    // this line throws file_not_found with Admin's token, or not_allowed_token_type with Bot's xoxb token
    const result = await client.files.sharedPublicURL({ token: "<Admin's xoxp token>" , file: file.id });

    // generate public url and return it
};