With WebSphere Portal 5.1 a new deployment option was introduced that allowed the installation of WP onto an existing node managed by a deployment manager. AFAIK the main reason for this new option was to simplify the cluster setup procedure.
Both options, installing WP standalone and federating it into a cell afterwards as well as installing it on an already managed node have different pros and cons each. In WP 6.0 the installation onto a managed node was even mandatory to enable WebSphere Process Server integration.
Surprisingly WP188.8.131.52 does not support the installation on a managed node anymore. All nodes of a cluster need to be installed as standalone systems first and then be federated into a WebSphere cell.
For WP 6.0 the installation procedure is described here. The 6.1 info center does not mention this procedure anymore and it no, it was not dropped accidentally.