The Business Source License is not an open source license. Open source does mean "free to use without encumbrance" - see points 5 and 6 of the Open Source Definition at https://opensource.org/osd
Approximately zero people who make real business decisions care what the OSI considers a "real open-source license" to be. They care what the text of the license says.
Also, many licenses, such as the GPL (one of the very first "open source" licenses), have certain encumbrances; you cannot redistribute GPL-licensed software without either including its source code or making it readily available.
