How are you pressing the cheese? Are you using a cheese press? I have done some traditional cheese making and it does take a lot of force. Not quite to the point of creating an atomic reaction though. For something simple, look for a Dutch press. It is a lever based press that helps. A screw press can also work.
Classic Chevre uses a bacterial culture as well as rennet to coagulate the cheese, but that is not the only way to make a goat's cheese. The recipe you linked to doesn't have a long incubation time, so I doubt there's any intention that the buttermilk is inoculating the cheese and there's no rennet. So I think the "bacteria" idea is a red herring.
A very simple goat's cheese can be made with goat's milk and lemon juice. I have had great success with it. It does make a fairly soft cheese, though how soft depends on how long its left to drain.
The recipe I use has 1/3 cup (US) lemon juice to 1 quart milk. I'm in the UK so this works out as 2 lemons per litre. The result is quite "lemony" and you might want to use another acid source (white wine vinegar for instance) but if you keep trying you can home in on what you like.
Simple acid cheeses like reasonably high temperatures. My recipe uses 180 - 185F (which may be hotter than you have used) before adding the lemon juice.
The other thing is, its generally much easier to just let the milk sit after acidification so the curd can develop. 10 minutes is usually enough, but you can always leave it 20 or 30 minutes if the curd is slow to set. The guilty kitchen recipe goes straight to ladling out the curds. That is something you do for a ricotta or high acid cheese (which may sort of be what they are aiming for - I don't cook with buttermilk myself so don't have the experience) but I'd want to let things set a bit first with a simple goat's cheese.
So: warm slowly to 180-185F, add lemon, sit, strain through cheesecloth to the texture you like.
Crumbly may be an indication that its not setting long enough?
Best Answer
You can use UHT milk in cheeses that don't contain rennet, basically cheeses that are formed by adding acid to milk, allowing it to curdle, and then separating the whey. Quark, Paneer, Queso Fresco and Ricotta are all cheeses of this type. Opinions differ on whether UHT milk can make good cheese of this type, but it's clear that you can achieve cheese. Serious Eats discusses the use of UHT in Ricotta here. "UHT milk does not work as well as regular pasteurized milk. It has a smaller yield, and the curds do not cling together properly. The results weren't terrible, and would do in a pinch."
Even though Mozzarella can be made without rennet, it has a reputation for being particularly troublesome with UHT milk. Pictures of UHT Mozzarella Fail
UHT Pasteurization denatures the proteins in milk to the point that they can no longer hold the curd shape, they can't fully solidify. So you can certainly try UHT in Ricotta type cheese, you may find it perfectly tasty, but I'd recommend not even trying to make rennet cheese or Mozzarella.