The antenna is for LoRa only and you can safely use Reach RS/RS+ without LoRa antenna on.
The requirement to have the antenna attached all the time comes from very high power radios, which generate a lot of heat without the antenna. LoRa radio inside Reach RS is really efficient and consumes only a a fraction of the power of typical radios in RTK receivers. Also, if you disable corrections output to LoRa in base mode tab the radio will not transmit anything at all.