There's no load in this part of the bike
Yeah, not so fast.
Any side-to-side load on the seat post is going to pull that crack wider, and the existence of that crack means those forces will flex the frame in that area in ways it's probably not designed to handle. That flexing will then put stress on the rest of the frame, likely concentrated on the inside of the bolt hole opposite the crack, again a location probably not designed to be stressed like that.
In fact, those very side forces are likely what caused the crack in the first place.
So we get back to
The crack has nowhere to expand to
So the front half of the bolt hole was designed to resist any stress that tries to pull it apart?
Just like the area in front of the seat post itself was designed to resist being pulled apart from side-to-side forces and not crack?
That didn't work so well, now did it? So why would the front of the bolt hole be expected to resist cracking, in an area less likely to be designed to handle that stress? The area between the bolt hole and the seatpost itself should have been designed to resist side-to-side forces and not crack - yet that failed.
How fast did you get your response? This might be a canned response to a design issue the manufacturer doesn't have the resources to correct. A fast reply implies the analysis and consultations necessary to respond were already done. I'd expect an actual analysis and reply to take at least a week, probably even more as I'd think it would have to be pretty well vetted on top of any engineering analysis.
I might be tempted to get a stainless steel hose clamp and some rubber padding and clamp the rear end of the top tube to limit any side-to-side flexing that could cause a new crack to form at the front of the bolt hole.