What does “That is a very strong historical point” mean?

strong does not mean:

“This is definitely true because it sounds convincing.”

It means:

The claim has substantial evidentiary or scholarly support and is particularly useful for the argument you are constructing.

For example:

“Formerly enslaved people remained economically disadvantaged because they lacked access to land.”

That is stronger than merely saying:

“Former slaves were probably poor.”

Why?

Because the first claim can be supported by historical evidence concerning:

land ownership,

labor arrangements,

tenancy,

sharecropping,

credit,

economic dependence.