Perhaps your Costco tech is unfamiliar with the ReSound smartphone app and how a smartphone like an iPhone actually works in conjunction with the ReSound app.
There are two volume controls available to you (I’m an iPhone user now, but before that, I wore ReSound hearing aids using the Smart 3D app on the Samsung Galaxy Note 8).
When you’re streaming, the app controls the volume of external sound in your ears via a slider as shown by the following iPhone screen capture.
Moving the slider to the left will decrease external sound, and moving the sound to the right will increase the relative external sound volume. Tapping the speaker symbol on the left end of the slider will turn it red, muting all external sound transmitted through your HAs. Tapping the double horizontal bar on the right end of the slider will allow you to adjust the external sound volume in the left and right ears separately. Streaming Focus and Hearing All are two presets that, given your current external volume set before you start mucking with the slider, either mute external sound a set amount relative to streaming volume (Streaming Focus) or make streamed sound and external sound ~equally loud (Hear All). This may have been the setting that your fitter was thinking needs to be adjusted in Smart Fit. Otherwise, he doesn’t know what he’s talking about.
The ReSound/Jabra app does not control the volume of the streamed sound. Either your phone hardware volume rocker/buttons control the volume of the streamed sound or a slider in the app that’s streaming sound to your HAs does, i.e., if I’m using the Apple Podcast Player, I can use my phone volume button (makes jumps in sound volume) or use the Podcast player volume slider to adjust volume in larger or smaller increments.
I’m a ReSound Omnia wearer, but if the Jabras are identical to the equivalent model year ReSound-branded aids, they ought to work the same way.