In answer to your question, yes you are wrong.
It is not just the edges that will cause these carpets to shrink. You need to look at why the carpet is susceptible to shrinkage, this will give you an understanding as to why to clean it with a minimal amount of water. Polypropylene is a water proof fibre, it does not absorb moisture, so any water you add to the carpet will if there is enough of it, go straight down to the backing. The polyprop fibre is woven to a jute backing which is stitched together. The water makes the jute backing swell causing the stitches to contract, which in turn causes the carpet to shrink.
There have been cases of Belgian Wiltons shrinking just through the moisture in the atmosphere just after they have been laid, this gives you some idea how sensitive they can be.
To find out if it is polyprop, take a piece of fibre and put it in a glass of water. hold it underwater and roll any air out of it then let it go. If it floats it is polypropylene. You can also do a burn test.
To be honest this is all basic stuff, you may benefit from going on a carpet cleaning course to bring yourself up to date with all the fibres and cleaning techniques etc.
Hope this helps
Derek.