Could or was able to

Hello.

After years of devoted work, Mr. Baily finally could get a raise in salary last April.

Shouldn't it be 'was able to'?

This is from my grammar book (Oxford Practice Grammar, Intermediate)
For ability or opportunity in the past, we use could or was/were able to.
To say that the ability or opportunity resulted in a particular action, something that really happened, we use was/were able to but not could.