Transformers are coupled inductors. They are specified by such parameters as inductance, winding ratios, frequency response(!), max current before core saturation etc etc. What transformers do is to CONVERT the impedance from one port to the other. In so doing they change the voltage as well. The Bartlett article contains the formulaes no? If not think conservation of energy (UU/R) between input and output.
You're interpreting the HOSA spec too critically. What Hosa tries to say is that if the load impedance is much lower than 50 kOhms
the core is approaching saturation (seen as brickwalling on the output side). The sbm-1 line in is probably well within what the Hosa can drive to specified levels.