Did you apply to a startup? Then they give you this reason of having to "wear multiple hats" so that they don't have to spend on hiring extra resources.
BTW what you descibed is the downside of the IT field whereas in say the healthcare sector a heart surgeon doesn't have to perform brain operations.
I have worked in quite a few companies and have friends who have worked in quite a few companies.
Basically it comes to this : Do you want to hire a dedicated Native mobile developer to just maintain your mobile container app in your hybrid-app structure where bulk of your app will be in JS/HTML and your backend is anyways in node.JS? True, sometimes you need some changes made to your container, for example that SSL pinning you wanted to add for security or handling push notification specifically but those are done once or twice or five times in one year. May be you can have someone who has spent most of their time in developing apps using web-technologies but are curious enough to get their hand dirty in Java or Swift with respect to Android SDK and Apple SDK / XCode. They 70-80% of time maintain your core aspects of your backend services but 20% time they also look at the container that runs your web-based app in a mobile.
See where it goes? Now when you are getting bigger and you need your backend guy to dedicate their time 100% on backend and you are going to add so many native features in your app, you hire a Native developer.
Now coming to the "healthcare" example : ALL the doctors learn basic anatomy, medicine etc. You GP is your generalist. So such a person is your primary health care provider. She can prescribe a medicine for heart burn or she can refer you to a gestro if symptoms suggest Berret's esophagus. Thats what most developers are : generalist with varying deeper experiences in different fields like backend server, mobile native, web-development etc. Most of them can do all of these stuff (with copious help from stackoverflow / google) though not fast enough or at times not scalable enough. There you need a specialist.
For example : Currently I am one in scaling backend apps, previously I was an experienced dev in native mobile development, before that I was developing Publish/Subscribe platform for enterprises and before that I developed chipset design behind PCI-express for a major semiconductor house. My degree interestingly was in IT but I was interested in EE during my university days. Over the period of time, specialization changes but your general abilities stay. So, if my boss comes and tells me that I have do a bit of DBA work or project management work for the time being, I will do it. Though not as well as a dedicated PM or DBA but decent enough. The understanding is mutual with my boss on it.