I just wanted to explore an alternate explanation.
Most people (in the comments) are assuming that your senior is wrong and that may well be true. But the opportunity I see is for you to dig deeper with your senior and find out why he/she believes what he/she believes.
One of the "7 Habits of highly effective people" is "seek first to understand, then be understood".
By digging deeper, you might learn something valuable even if you don't agree with his/her position on testing. At the very least you might learn what kind of argument or proof would convince your senior to give unit testing a try (or to let you try it).
The funny thing is that in the following weeks he proudly announced covering an important part of application with unit-tests.
I guess the point was mainly about how important it is to think carefully about your design decisions and the changes you make to the code base. And that 100% coverage is a waste of time. With both points I agree. Maybe the rest was actually a joke. :)
Interesting. Now I'm even more curious to know exactly what he meant originally.
Do you think you'll ask about it or let it go?
I think I will leave it to be a mystery :)
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