The word “just” in your comment disgusts me
They're suggesting you spend a minute or two per month thinking about it, not meticulous tracking.
That might not be practical, but what they are describing is a perfectly good use of the word "just".
So no, "just" is hardly fair.
Correct, they are suggesting basically just guessing.
Which is why "just" is correct for their suggestion.
It's not a good suggestion but it really is that easy to implement.
When I was doing it, I worked in an actual startup and granularity of time allocation was in weeks. This week I was doing the thing, the other I was mostly doing bugfixes/refactorings etc.
You could do more precise and account with hour or minute granularity with tools if you have to