Why is the TP4056 IC called a "linear li-ion battery charger"
The TP4056 uses a pass transistor to regulate the charging voltage / current to the battery.
That pass transistor is controlled in a linear way, meaning a gradual on/off control. So the pass transistor can be fully on, fully off or any state in between (having a certain resistance).
A different type of regulator is the switched regulator where the pass transistor is switched so it can be on or it can be off. There is no state in between (like a high resistance state).

Or is it linear as in &quot;linear power supply vs switching power supply&quot;?

Yes it is.