Using 有 vs 是 to describe an object
有(一)点 as a set can use to mean &quot;a little too...&quot;. See this Chinese Grammar Wiki page.
Some examples from the page:

我 有点 饿 。
I'm a little hungry.
这个 菜 有点 辣 。
This dish is a little too spicy.
昨天 有一点 热 。
Yesterday it was a little too hot.

是 does not have this usage, so 他是一点儿严厉 is wrong.
However, you can say 他是有一点儿严厉, where 是 is used for emphasis.