The justification is spelled out literally few words down the line - to unify the visual language.
If that is not clear, here is an example of Microsoft Office's excellent execution: https://medium.com/microsoft-design/redesigning-the-office-a...
If that is not clear, here is an example of Microsoft Office's excellent execution: https://medium.com/microsoft-design/redesigning-the-office-a...
So, if the idea behind icons is to quickly locate the tools you need, even if you are not super-familiar with the tools, I think the previous icons served that purpose much better.
Even if I had memorized what every icon means, I can't tell them apart without looking closely because most have the same shape and similar colors. "Everything looks the same" is not good design.
For starters, each product has a distinct icon color, which in and of itself makes a huge difference in usability and the ability to quickly determine which icon to click. This is something the Google redesign fails completely on.
Secondly, every element behind the single-letter in each MS icon has distinctive elements that are related to the product. Excel actually has grid-cells to mimic worksheets in a minimalist fashion. Powerpoint has the pie-chart. One-Note has the fantastic minimalist binder with tabs in the background.
Between the unique color and graphical elements associated with each product, there is plenty that helps a user easily distinguish one from the other while they all still have a cohesive aesthetic.
I find it to be an outstanding redesign. In comparison, the Google redesign basically has almost none of the benefits above and makes it a lot harder to distinguish icons at a glance.