I agree with you based on my experience. I've never tried to solder those wires because I can feel the hardness of them to know they aren't copper and by feel, must be stainless steel. But my experience isn't everyone's and perhaps it is possible there are oxygen sensor cables out there that aren't like mine.

But I'm not sure I agree with your "cheesy" characterization of the universal splice kits. Well maybe cheesy. I've used them twice in years past (not on Volvo) and was impressed by the physical security and ability to exclude the elements, or at least that's the impression that got left in my memory. But of course, having the original harness length and connectors does a better job if affordable. No one wants electrical connections (splices) exposed to the road under the car regardless of how well encapsulated they appear to be.
--
Art Benstein near Baltimore

Pilfered from Eric:

The Parts Man Prayer

I work behind the counter
In an auto parts store
Sometimes I'm called a genius
Sometimes I'm called much more.

I say I'm no mechanic
But when their job goes sick
Everyone comes in and asks me
What makes the damn thing tick.

I'm supposed to know the number
Of nuts and bolts and gears
For every machine ever made
For more than a hundred years.

I'm supposed to have the answer
For everything unknown
To every Tom, Dick, and Harry
For all their questions thrown.

My life would be a pleasure
And I'd grin from ear to ear
If only they would tell me
Their Model, Make, Year