Like the author, I would use the singular.
Use of singular and plural in this sort of context (pairwise relationship) differs between English and German, but what is idiomatic in English may not be easy to define.
I would suggest that the singular may be preferred when the noun (here: teenagers) is used generically, whether singular or plural.
If the text had been about specific teenagers, I think the plural form would be preferred (even if each teenager was immersed only in his or her own smartphone), e.g.
"The two teenagers flopped down on the sofa in the living room, ignored the main TV set, and immersed themselves in their smartphones.
If the singular were used in that context, I would think they had one smartphone between them.
This probably has something to do with bound variables (for each x, x immersed x in x's smartphone), but the rule may be rather complicated.
The plural, may, of course, be ambiguous (not necessarily pairwise) .