Steps to reproduce:
1. Open a new project
2. Import attached Adam model (it's a lot smaller than downloading the whole thing)
3. Select Adam model under Assets\Adam Character\ Adam
4. Switch to Rig tab in the inspector
5. Observe the Biped warning

Expected result: No warnings are shown
Actual result: A Biped warning is shown

Reproduced with: 5.3.0b1, 5.3.0f1, 5.3.8p2, 5.4.6f3, 5.5.5p1, 5.6.4p2, 2017.1.2p3, 2017.2.0p3, 2017.3.0b11, 2018.1.0a4
Not reproduced with: 5.2.5f1
Regression in: 5.3.0b1

Note: Due to the regression being so old, I could not find 5.3 alpha releases