I'm replying to myself here.
After posting earlier, I went on reading some other posts in the forum and came across a couple of others with problems similar to mine with that is_executable variable. OK, I thought. I'll just double-check what I've done and attempt to spider my site one more time.
I refreshed all the phpdig database tables, then attempted to spider my site again. This time, the spidering page showed up like this:
For what it's worth, I ended up having to drop that table and then re-create it to get the phpdig database refreshed so I could try spidering again.
Now I'm sure that I have that is_executable variable being set properly, so what is the problem this time?