Please don't scare others by this misunderstanding. CIC always wants your complete application therefore it not understandable if they PUSH first come first served for sending applications, remember on invitation letter they mentioned must receive on or by June 28, 2019, if they accepted only 20k complete applications arrived first, they will clearly mention it.
You got the invitation means you got your spot. your spot will be transferring to someone else ONLY your application is not eligible. Eventually, the application is incomplete. they may ask for additional docs instead cut your spot
Think you misinterpreted what they are saying. 27000 people got accepted by EOI and everyone seems to have been given an invitation. CIC said they will only be processing 20000. Seems as though they will process applications from the first 20000 EOI and process the other 7000 as applications don’t qualify.