It's possible, but unlikely.
It's possible
Cakes cook from the outside in. If you are not careful you might pull out a cake cooked outside but raw inside. This happens when the oven is too hot.
What to do if my cake seems ready from the outside (it's brown as it should be) however it's not done yet inside? [...]
If you find that your cakes are brown on the outside but are still raw on the inside then it is likely that the oven is too hot.
ASkNigella.com - Baking a cake
In fact, there are cakes which are cooked that way on purpose, for example the chocolate lava cake
Furthermore, even in the case of full cooking, internal temperature rise to less than 100°C, which are high, but not massively outside the phone's operating range.
I've never seen any recipe that looks for an internal temperature of over 200F (93C). Bread is generally "done" at 190F (88C). At temperatures at or above the boiling point of water, you're getting into candy making territory
What is the maximum internal temperature of baked goods?
One should store an iPhone between -20°C to 45°C. This is what recommended by Apple and what presumably guarantees minimal to no chance of breaking. It's not unconceivable that phone might survive to temperatures of twice that range.
Store the device where the temperature is between -20º and 45º C (-4º to 113º F).
Keeping iPhone, iPad, and iPod touch within acceptable operating temperatures
In fact, the phone in the picture shows the correct screen that should appear when an iPhone has overheated (same source as above):

It's unlikely
While it is possible, it's unlikely because checking whether a cake is cooked is very easy and therefore is commonly suggested. The technique is inserting a toothpick and checking whether it gets wet or dirty. This would have revealed the phone, of course.
[...] How do i really know that my cake is done?
[...] You can also check the cake by inserting a thin skewer, cake tester or cocktail stick (toothpick) into the centre of the cake. If the cake is fully cooked then the skewer will come out clean, if there is raw or partially cooked batter clinging to the skewer then return the cake to the oven for a further 5 minutes and test again. For rich fruit cakes the skewer test is the best way of checking that the cake is cooked all the way through.
ASkNigella.com - Baking a cake