As iegomez already explained, join requests are sent based on the behavior of the firmware alone.
It’s a little unclear if Class C necessarily implies OTAA. The following passage in the LoRaWan spec could be read as lightly assuming that it does:
Note: There is not specific message for a node to tell the server that it is a Class C node. It is up to the application on server side to know that it manages Class C nodes based on the contract passed during the join procedure.
But there does not appear to be any actual technical reason forcing that. The only “interaction” for setting up class C are some new MAC commands.
So really this seems to come down to the assumptions made by whoever wrote your node-side firmware.
If this node is sending join requests, then it is not correctly operating as ABP
You could confirm class C behavior by seeing reception of RX2 downlink transmissions outside the normal time limits, or probably better by adding logging into the node firmware to say when it enters/exits that extended RX2 mode.