That quote pertains to the polling engine, however WMI is more taxing on the end node compared to SNMP as well. That said, it is still generally negligible and shouldn't impact system performance. How much will vary by node and what information is being returned.
So if I have 1500 windows nodes that I want to change from SNMP polling to WMI, I can expect an increased load on the polling engines
polling every 5 minutes
SAM templates in play on 500 nodes (average 3 components).