Yes the battery is sealed inside and I have looked up on YouTube to find out how hard it would be to get the battery out and replace it but since the phone is so new apparently there are no videos out there yet.
But the battery size seems to be very large considering that it's a relatively low end phone and I have had very good battery life from the phone.
I don't think that me and my phone are joined at the hip but just this morning I did use my phone a fair amount and I started out with 93% and I'm now at 70% I actually started around. 7:30 in the morning and it's now 1:41 in the afternoon.
To me that is very good battery life so it seems that they're not exaggerating about the battery life. Personally I would not let that deter you from buying the phone.