The verb 捕まる・つかまる is, according to jisho, an intransitive verb. Most of the study questions that accompany 捕まる (JLPT N4) | Bunpro use it in that way. For example:
叔父が先日、信号無視で捕まったそうです。
I heard that my uncle was recently caught jaywalking.
However, the definitions also include “to grasp”, and this study question uses it in a transitive way:
迷子になると困るので私の手をしっかり捕まっていてね！
Hold on to my hand tight because I don’t want you to get lost!
手をclearly marks a object that is being grasped.
I don’t get this…
Indeed, を can be used for intransitive verbs. You can see a reference with four separate definitions of を here: A Dictionary of Japanese Grammar (search を in page). I’m not confident enough to promise the usage you found is correct, but it does seem in line with some of the use cases I found on google at a cursory search for using を with intransitives (which are absolutely not the same thing as English).
Hm. I do see that を can be used to mark locations that motion originates from or moves across, or as the cause of some emotional reaction, but none of those cases apply to this sentence.
Searching for uses of を with intransitive verbs did turn up Japanese Transitive and Intransitive Verbs at Tofugu, which does go into some of the subtleties of what makes a verb transitive or intransitive, and had some interesting examples. Most of them concern the same intransitive motion verbs that were relevant to the uses detailed in the Dictionary of Japanese Grammar, though.
Now here’s something else interesting — looking at examples of sentences that use 捕まる, I’m mostly seeing に as the particle being used to mark the thing being grasped — I don’t see any using を.
There are definitely nuances that make the usage of を and が a bit tricky sometimes when it comes to transitive and intransitive verbs, but in this case it appears to be a simple error. As far as I am aware, only に will be used with 捕まる. After confirming with @Haru, it looks like it may have been written as if the verb being used were つかむ, not つかまる. We’ll fix this one asap!
Thank you!