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
OR they need to maintain a set of activity codes and a timesheet outlining how many hours (or partial hours) each week are spent on what types of tasks.
It's unnecessary complexity if you want to be in actual compliance with the tax code vs just guessing whether XYZ task is on one side of the line vs the other and hoping it doesn't come back to bite you later.
What about HR, etc who use excel documents?
IF they are using it rather than developing it, no. If they put in 5 hours a week writing code, yes for those 5 hours. This isn't hard.