The slash was just because MarkDown doesn’t really make mathematical notation easy. The point is that with a slash, the 6 is over the entire 2(3) divisor. It’s the difference between these:

You can even see that the automatic solution (in yellow) parses the two differently. In the first example, it correctly resolves the 2(3) first, because you should always simplify both the top and the bottom as much as possible before you resolve the division. But in the second, it parses the 6÷2 first, because it is left ambiguous. The slash is literally the horizontal bar, putting the dividend above the entire divisor. Except it’s in a single line, instead of taking up three lines of text for a single operation.

I believe they do mean the fact that subtraction is just adding the negative and division is just multiplying by the inverse. You can look up field axioms to see how real arithmetic is really defined. It's much more convenient to have two operations instead of four.

Addition asks "What do you get when you combine these two numbers?"

Subtraction asks "What do you need to combine with this number to get this result?"

Multiplication asks "What do you get if you add this number to itself this many times?"

Division asks "How many times do you need to add this number to itself to get this result?"

In many ways, all of these operations are syntactic flavor for addition. Subtraction is addition in reverse. Multiplication is repetitive addition. Division is repetitive addition in reverse. Exponents are recursive repetition of repetitive addition. And so on.

Look into the axiomatic proof of 1+1=2. It will shed some light on how mathematics is just complex notation for very, very simple concepts at scale.

The other commenter is correct, but another way to think or visualize is that any subtraction or division operation can be understood as an addition or multiplication.

X - 5 = X + (-5)

X / 5 = X * (1/5)

You can think of subtraction and division not being distinct or separated from addition and multiplication; instead, they're just a shortcut notation in mathematics because everyone was tired of having to write extra characters.

Figuratively, at least.

There's a reason why the conventional division symbol requires grouping its terms.

If you see an exercise like that, the exercise is bad and your teacher must be educated. Now, try putting that into a computer language and see what comes out.

tightly bound visual unit

I think you nailed it on the head. The expression isn't technically ambiguous, there's exactly one solution, and neither is the notation incorrect, just unconventional. In this case though, forgoing convention makes the expression typographically misleading. Hence a reason why we have these conventions for writing out expressions in the first place: to visually reinforce the order of operations thereby making expressions as easy to read as possible. So it's not written wrong per se, just unnecessarily confusingly.