To display the LAS as a TIN you will have to use the conversion tool LAS Dataset To TIN
Just some additoinal info on support scene layer types
3D polygon - 3DObjects generated from multipatch features, can be published to AGOL from slpk or from a hosted feature layer
point - Points will have autothining applied, can supported millions of features, can be published from slpk or from a hosted feature layer
3D point cloud - latest scene layer type, published from Pro 1.4 using the create slpk tool.
Additional scene layer
Integrated Mesh - published from Drone2Map using an slpk generated from the app.