I didn't find a way to link ISY to SmartThings. What I did was use the Join An Existing Network feature in the SmartThings App, which apparently didn't work. I believe I remember doing that with the previous Z-Wave network and it working well, but I may have that wrong. Memory fails the aged. I'm capable of overlooking a single step and ruining the process.
Apparently I should proceed with the approach recommended in the Link your provided. What I want to avoid is any extra verbiage such as required in the Amazon Skill Sets requiring you to say "Alexa, tell ISY to tell . . ." I did that with Echo talking to CQC and found that Alexa has trouble understanding that many additional words.
I'm using the Insteon Hub with 3 bedroom lamps and the minimal verbiage required gives me almost 100% reliability. All I have to say is "Alexia, bedroom lights 80%" and it works every time. No need for adding "turn on the". Simplicity with Echo is the key to reliability, IMO.
My replacement Z-Wave dimmer for the one that slowed the Z-Wave down is due today. Once that's installed I can move onto integrating ISY with Echo. I like to do one thing at a time so when I hit a problem, I know exactly where it occurred.
BTW, I did learn how to create a Secondary Controller with ISY. I was able to make my Cooper Aspire a Secondary. Seems to work out well.