The problem is that this question is not as simple to answer. You can respond: yes it is supported. However, that is under circumstances.
The problem lies in the request for 802.1X. That question seems simple, but the reality is that 802.1X can mean almost anything.
If you want to do EAP-MSCHAPv2 against your active directory (that uses usernames and passwords under the hood), you can't, as via LDAP you can't support MSCHAPv2. That is where the statement: 'You need to have an external RADIUS server for that' originates from. There are other reasons why you should avoid MSCHAPv2. But that question is typically not asked.
If you implement 802.1X with EAP-TTLS, you can connect via LDAP without RADIUS and use AD passwords. I did not find a definitive answer, but it seems that the same server certificate validation problem exists with that method.
Which leaves us with that you probably want to implement EAP-TLS authentication with client certificates, which opens another can of worms if you don't have the proper backend infrastructure. But you can issue client certificates to your clients and have the controller validate them, even without an LDAP connection to Active Directory.
ClearPass will make your life much easier in many ways, and many of the basic authentication things of what ClearPass can do, can be done with another RADIUS server as well. In my experience, after spending lots of hours with the Microsoft built-in RADIUS server, people like the visibility, scalability, and add-on features that ClearPass provides as additional value over the 'free' approaches that come with troubleshooting and 'find out yourself' research costs.
So to summarize, you question can be answered both with a 'yes' as the formal answer, or as a 'no', 'it depends', 'possible but not advised' in most real-world scenarios. Then of course, it depends on the partner to do the proper investigation and provide an honest answer, and the customer to ask the right questions and define the right expectations.
Hope this provided some light on this topic.