I think that it means that thing #1 and thing $2 might appear to be just as difficult to untrained eyes while one would require very little work because of what's already available (libraries, data or whatever) and the other would be a very large task because most of the work hasn't been done yet (but of course a different engineer might be able to find more efficient ways to do things)
Comments
You are taking the xkcd strip very literally instead of understanding the central concept on it.
You comment reminds me of this: http://www.uh.edu/engines/epi879.htm
I think the central concept is believe a smart engineer when he says something can be done, but not always when he says it can't be done.
I think that it means that thing #1 and thing $2 might appear to be just as difficult to untrained eyes while one would require very little work because of what's already available (libraries, data or whatever) and the other would be a very large task because most of the work hasn't been done yet (but of course a different engineer might be able to find more efficient ways to do things)
Just pointing to the fact that 'Virtually impossible in CS' is a concept that changes over time. The better the context the better the joke.