I don't know if the manager/senior dev will take that post scriptum as a show off or as a helpful tip?
I think that you should refrain from saying so, at least for now.
Currently, you are not hired by them (although you may be in the future), so it is not your responsibility yet to evaluate their website.
Mentioning such thing may come as a bit arrogant or they may not take that feedback too happily, and is clearly out of focus for the application process; these things can affect your application in a negative way.
If/when you get hired, then you can mention such things without problem.