There are two good reasons for this.
1) Girona being successful (particularly on a modest budget) is an endorsement of CFG's efficiency. The media have spent years decrying the multi-club model. It's not renowned as an industry which attracts people with enough let's say, humility (read, emotional intelligence if you prefer) to admit when they have been wrong.
2) The second reason is insurance against having to recant in in the case of Girona being successful.
Repeat a lie often enough and it becomes a truth. If they say enough times that both City and Girona can't compete in the same competition, then when (well, if) it happens, the clear inference is that those pesky Johnny Foreigner types wot don't look like uz have been up to something corrupt again.