It is just as you guessed. In Thumb-2, when loading a 32-bit number it is often using a MOV/MOVT instruction pair, e.g.
MOVW R1, #0x1234 ; Set the value of R1. R1 is now 0x1234
MOVT.W R1, #0x5678 ; Set the top-16 bit of R1. R1 is now 0x56781234.
IDA Pro recognized that the combined immediate value matches the address of a selector, and uses the :lower:
and :upper:
syntax to indicate that the value is split into two 16-bit parts.