A person is identified, if the ID references only one user in the whole dataset[1]. This also makes any information linked to the ID PII.
the ID would be pseudo-anonymous if one would need some extra data, to which they don't have access to, for linking the ID to one specific user in the whole dataset[2].
So to answer your question, RTB ID is not pseudo-anonymous as it only references a single user out of all of them.
[1] It's also important to understand the definition of PII in GDPR context. Which is any data that relates to an identified or identifiable person. Identifiable is the same as distinguishable. Knowing this helps to understand where the line is. https://www.lexico.com/en/definition/identifiable
(5) ‘pseudonymisation’ means the processing of personal data in such a manner that the personal data can no longer be attributed to a specific data subject without the use of additional information, provided that such additional information is kept separately and is subject to technical and organisational measures to ensure that the personal data are not attributed to an identified or identifiable natural person;
A person is identified, if the ID references only one user in the whole dataset[1]. This also makes any information linked to the ID PII.
the ID would be pseudo-anonymous if one would need some extra data, to which they don't have access to, for linking the ID to one specific user in the whole dataset[2].
So to answer your question, RTB ID is not pseudo-anonymous as it only references a single user out of all of them.
[1] It's also important to understand the definition of PII in GDPR context. Which is any data that relates to an identified or identifiable person. Identifiable is the same as distinguishable. Knowing this helps to understand where the line is. https://www.lexico.com/en/definition/identifiable
[2] Definition of pseudonymisation, 5'th bullet-point: https://gdpr-info.eu/art-4-gdpr/ sheds some light on this.