Custom login data security

Hi Codans

For some reason I would like to collaborate on a doc without asking users to create a coda account.

Using forms for interactions

I made a conditional logic on texts and tables to only show data to the users connected with my light custom login (see the doc)

How safe is this method ?
Can someone confortable with API’s access the hidden data ?

The attached doc is just for illustration and does not contain any sensitive data.