With the work being done a few weeks ago, there is nothing wrong with calling the installer who tiled and asking why grout was used in the changes of plan instead of caulk. Ask about the corner grout cracking. What if? Will remediation be offered?

Caulking a shmear of caulk over a grout joint is not going to prevent the grout underneath from cracking if the grout is prone to cracking. Some corner grout never does crack. With your house being an "old 1060s", it may be nicely settled in. You may not have differential movement between the wall planes that leads to corner grout cracking.

Grout sheds water. Caulk sheds water. Both are superficial things that "pretty up" tile installations. The real waterproofing should be behind the tile.

Caulk needs some depth to it, if it's just a veneer over grout it may peel delaminate over time.

Latex
caulk versus silicone? Both will shed water fine, but 100% silicone will perform better in
standing water than 100% latex.

You'd be correct to make a phone call and to ask questions. And to get answers. Sometimes I'm of the mind that although best practices may not have been followed, second-best practices can still turn out fine. Meaning, you could leave the installed latex caulk until you have issues. You may never have issues. But if you do, then replace it. Same with the grout in the corners. Leave the grout in the corners until it cracks. Then replace it.

When you talk to the installer, if he offers to come over and remove the grout and replace it with caulk? Sure, you could take him up on it. Or just get an assurance from him that if problems develop down the road, he'll swing by to make things right. It's best to get those assurances in writing.

It's really what you feel most comfortable with. And how accommodating the installer is with regards to backing up his work.

Being that it's an older home, it actually has more chance of moving dues to seasonal expansion and contraction. I agree with Mongo. Call your installer.