That's perfectly understandable to me Janet. The west was interested in hara and not Koshi.
As I said it was of no significance to the Japanese for they were brought up knowing it. What someone like Tohei would see and note is the emphasis those from the west put on upper body and force thus the principles are to let go of all that and learn centre and relaxation.
One point leads to the stillness of mind.
Relaxing, weight underside leads to the understanding of Koshi.
Extending Ki leads to the understanding and use of Hara or centre.
I must say that I too never heard Tohei emphasize Koshi as such but did explain such things as dropping centre. When you see him do such things and watch carefully you will see it's not from the front so to speak but from Koshi. The west tend to put all this down to a low centre of gravity and look no further.
But Tohei Sensei was a very clear, plain-speaking teacher. If he meant "koshi" he would have said "koshi."
I will grant you that by "centre" (as in "drop one's center") he probably did not mean the same thing as "one point" (as in "keep mind at one point") or he wouldn't have used a different word.
I can't think of anything in Tohei's teaching to suggest it is the sacrum/sacroliliac area.
In my own practice, I experience the "centre" as being that entire area between the diaphragm and pubis, encompassing front and back, a large globe if you will, resting on the hips, with the one point within it.
I still don't get the "koshi" thing outside of your system but that's ok - I think we have each clarified our language and usage and may have to agree to respectfully disagree.