The proof that the business model works and/or scales cannot come from a competitor (except 100% was copied from them but then again just cloning is a dumb idea anyway). That proof is the goal of any serious entrepreneur. I don't see a convincing argument towards VCs here.
Examples: "We are facebook for bicycles!": that's just a facebook group. "We are Heroku for PHP/Clojure/Python":http://blog.heroku.com/archives/2011/8/3/polyglot_platform/
These are my best examples, but if X is just a feature that Y could implement or acquire, then making that comparison could be a major mistake. If they expand into your niche, then your X of Y idea backfires on you hardcore.
Edit: I saw that my variables were reversed at some places.
Very true. This could be the case whether or not you publicly call yourself "The X of Y." I've always though that most companies fall in that category, even if they don't know or outwardly admit it.
So basically, we're all in danger no matter what.
Unless, of course, the feature that Y could acquire is yours and you crush it...then your position is pretty damn awesome (assuming you don't mind working for Y).
Unfortunately, I still hear people describing us as "Airbnb for X". Comparisons like that stick, and if your business changes, it's hard to change a facile comparison like that.
This guy, to me, just unintentionally summed up all that is wrong with certain, vocal, sections of the startup community.