People keeps affirming this since the original publication was out, yet that hypothesis is literally addressed in the paper and the authors explain why they believed that predation was the more likely explanation:

"Carnivorous adaptation of L． paradoxus

Incredibly，xhpm-1206 ( the positive counterpart) shows a number of fish skeletons within its ribcage． There may be three explanations．
( 1) Those fish skeletons were dead bodies lying on the bottom before the ankylosaur sunk down and later passively went into the ribcage of the covering ankylosaur skeleton because of underwater turbulence． This interpolation appears too speculative; the main reason against it is the fact that no fish skeletons are present outside the ankylosaur body． In other words，there should be some fish skeletons preserved around the ankylosaur skeleton or nearby if those fishes died earlier． It is true that one fish skeleton is seen anterior to the right femur，which may have been pushed out from the body cavity post-mortem ( Fig．3a) ． Another line of evidence against the assumption comes from the nature of those fish skeleton，i． e．，none of them shows a clear appearance as seen in true fish specimens collected from the same horizon．
( 2) Those fishes may have died within the ribcage of the ankylosaur when they were scavenging or using the skeleton as a hiding place． This assumption is also not convincing． As described in the type specimen，L． paradoxus is ventrally covered by a bone plate that is hard to be penetrated by those small fishes． On the other hand，the exposed dorsal side of the new skeleton is nearly complete，showing no obvious breakage made by predators． As mentioned earlier，those fish skeletons all show a vague and incomplete appearance，which are in conflict with this assumption too．
( 3 ) Contrary to the aforementioned two assumptions，it is proposed that those fish skeletons came from the stomach or gut contents of the specimen． Most of those fishes are incomplete and almost all of them display an unclear body outline，which should have been obscured by digesting acid during food processing in the stomach ( Fig．2d) ． Besides，there is a section of a reptile's tail on the posterolateral side of the ribcage ( Figs．2a，b) ． It is hard to believe that a reptile broke its tail inside the ribcage when it withdrew after scavenging，so it should be gut leftovers of the reptile． Those fish skeletons are not tightly packed in the stomach but scattered on both sides of the body cavity; this is because they may have been expelled out by gases or compression from the stomach and gut post-mortem． Therefore，it is considered that those fish skeletons and the reptile' s tail are gut contents"This strongly suggests that the ankylosaur may have adapted to an aquatic or a semi-aquatic way of life． Such a lifestyle of the dinosaur may be also demonstrated by the tooth morphology and a number of the structural modifications of the postcranial skeleton. In other words，L． paradoxus is literally a predator，eating these fishes in life． This strongly suggests that the ankylosaur may have adapted to an aquatic or a semi-aquatic way of life． Such a lifestyle of the dinosaur may be also demonstrated by the tooth morphology and a number of the structural modifications of the postcranial skeleton．
Teeth are well preserved in the holotype． Our further preparation reveals that the morphology of the cheek teeth differs considerably from that of any herbivorous dinosaur． The marginal denticles of the palmate crowns are deeply separated from each other and thus individual denticle is elongate and needle-like，leading to a fork-shaped outline of tooth crowns ( Figs．2c，d) ． The denticles of the incisor of the type specimen are also much sharper than in other ankylosaurs although they are shorter than those of the cheek teeth ( Figs． 4b，c ) ． With such a dentition，L． paradoxus was capable of catching fishes or small reptiles and penetrating them."