You're absolutely right. Here it is:
http://lermagazine.com/article/return-to-football-after-achilles-tendon-rupture
Relatively small sample, but basically 2/3 of players came back at all, and most aren't as effective. They're judging effectiveness by games played and by certain "power" traits. BUT the big thing that sticks out to me is that the average age of those participants in the data was 29. Interesting question is how many 30-31-32 year olds are as effective as they were at age 29 in the general NFL population -- i.e., I'm not seeing anything about a control group here.
More interestingly (to me) and positively, there's a more recent study here:
Most football players returned to NFL after Achilles tendon repair
I'm not seeing the source so there could still be some issues. Big change, though is that while the overall return rate is only slightly higher (73%), they did do a control group analysis and found that for every position except linebacker, they couldn't find any proof that it leads to less effectiveness for those that do return.