The product description says:

"Normally closed specification technical parameters

Output mode: BNC (normally closed)"

Which implies that it doesn't depend on a grounded tool/spindle but has a contact which opens on tool touch. This would make sense as it would be compatible with most other probes/setters and machines. Why it says "BNC" I don't know as the connector is nothing like a BNC being a screw lock 4-pin type.