As long as you removed all the leaves, stems and debris (like you would do by using tongs and a mesh strainer) you should be OK.  If not, just do that and boil it again for a few minutes.  For that matter, why not go ahead and take it to boiling again and let cool anyway?

I have never had oleander soup the constency of chicken soup except during the process where I was boiling it way, way down to a syrup consistency to use in a skin cream.  Consider that the finished product after boiling and straining only weighs 1.08 the weight of water.  That is mighty thin soup.  I really don't know why the guy who figured out how to make an extract modeled after the Ozel patent for Anvirzel decided to call it soup.  It is not nearly as thick as soup and you only take it in doses less than a teaspoon (starting out) to 1 tablespoon at a time.

It should have been called oleander extract!

Here is a tip:  I firmly believe that the more straining the better in terms of avoiding side effects such a loose stools (or outright diarrhea).  The people who make the patent medicine report virtually no side effects from oral administration and the same is pretty much true for those I know who have done extra straining with the home made extract.