If you get swallowed by a Tarrasque, your only way of getting out is to deal it enough damage to force it to regurgitate you (60 damage or more on a single turn).
There's no mention of the AC when attacking from the inside of the monster. I'd presume that means that the AC is the same at the AC on the outside of it, i.e. 25, which is stated as "Natural Armour". This doesn't seem right though, because there is no natural armour on the inside of a creature, at least nothing as tough as the creature's carapace. The inside of creatures tend to be soft and vulnerable.
I guess this is just a juxtaposition of game mechanics vs game flavour, but I'm pretty sure that if my fighter somehow swallowed a creature, it sure as heck wouldn't be using my AC of 26 for the attack rolls!
If you attack a Tarrasque while swallowed, what AC do you need to beat to hit it?