It wouldn't be disingenuous at all.
Syria and Iraq are in the news a lot regarding refugees and ISIS. It would not be disingenuous to suggest the authors, rather than imply these two Asian countries may join the EU, are suggesting Turkey about to join is bad news because it boarders Iraq and Syria and would provide an easy access into Europe for said refugees and ISIS.
I agree with what you say in that there is no intention to suggest Syria and Iraq are going to join the EU but the message seems to be clear in that letting Turkey in means the borders of the EU are now right next to those countries which we know are areas occupied by Isis and, ergo, terrorist.
What is disingenuous is to imply that Turkey is about to join the EU, which the are clearly not. That is a lie and so is the £350m per week figure. However, that document is not aimed at people who might check the facts, it is aimed at people that just read it and believe it because it panders to their fears, real or perceived.