Well, even if Kurt Angle made his way back to the WWE, they wouldn't need to call him a thirteen time world champion, because he simply hasn't won thirteen world titles. If I'm counting correctly, he's had one WCW Championship, one World Championship, four WWE Championships and three TNA. Simple maths brings this total sum to nine world championships. And TNA probably count the IWGP championship as one as well, so we're at ten. I'm not quite sure how it works, but it's not giving TNA any credibility when they have to add non-existent title reigns on to their wrestlers when they're already accomplished enough.
One thing that happened with WCW when an influx of wrestlers came over from the WWE before the Monday Night Wars was the fact that WCW would credit these people with their WWF world championship reigns without actually saying where, how or when they won it. I remember within weeks of Randy Savage making his debut on WCW Saturday Night, WCW were calling him a former two-time heavyweight champion and left all mentions of it at that. Then when he won his second WCW championship from Flair in that brilliant match on Nitro, he was a four time world champion. Obviously, the WWE never did this while in competition with WCW, but once they purchased them, they actually acknowledged their world title was on equal status with theirs.
Obviously what's funny here is that, when Ric Flair joined the WWF in 1991, they never made reference to him being a former eight time world champion, but simply just the real worlds champion. When he won his WWF titles, he was merely a two-time world champion, and not the ten-time world champion which in reality he was. But when he came over to the WWF ten years later, he was a sixteen-time world champion. So them title reigns they decided didn't matter ten years prior now did. And that's because they owned them.
If WWE owned TNA, then no doubt they'd credit former TNA champions in their company as former world champions, but while in competition, they just aren't going to mention it at all.