I noticed that something is actually installed on remote nodes, is Network Inventory Advisor truly agent-free?

Solution: It surely is agent-free, however, it features several scanning methods, and by default in case the one that uses an RPC/DCOM agent fails, an agentless method is used. You can change this behavior in Settings. Please, note that scanning using self-deployed agent will result in more accurate results, as some of the data categories are inventoried in a custom way, bypassing WMI services.