How to understand the schematic symbol of this isolated, filtered BNC connector?
From datasheet.  A is shielded conductor.

Symbol is IEC60617.  From IEC60617 .


Coaxial plug and socket
If the coaxial plug or socket is connected to a coaxial pair, the tangential stroke shall be extended on the appropriate side.



Female contact (of a socket or plug) Socket.

So Molex's schematic symbol is a blending of the alternative input connection (03–A2–01) with coaxial socket (03–03–15).
A is the central contact, B is the BNC connection, C is the connector chassis and compression spring.  C is isolated from B by an integrated capacitor [9.4nF] for RF chassis-bonding (OP).