I agree with him. They always had like 5 different types of arrow that had various easily forgettable meanings.
Not just forgettable, but vastly different meanings. And these meanings have changed between versions. You really have to get close to the spec to grok what is actually being conveyed.
The devil is in the details - if you have to understand the details like the right arrows for relationship types and visibility modifiers, then it gets much less trivial.
And if you're going to omit those details, then do you really need a standard for your adhoc boxes and arrows diagram?
You can add a legend at the side of the diagram, problem solved.