English gives you options.
I can be walking down a street and suddenly realize that (the) houses on this street are old, and I can state my realization using either NP, that is with either
houses on this street are old
the houses on this street are old
neither is more correct than the other. the first utterance uses a bare plural noun phrase (houses on this street). this does not mean 'all houses on this street'. it expresses a generalization based on several instances: I have noticed that there are instances of houses on this street being old, and I state a generalization to that effect.
(again, I am not claiming that all houses on the street are old.)
the second utterance uses a plural definite noun phrase (the houses on this street). it refers to some undifferentiated set of houses on this street.
it does not have to mean 'all the houses' on the street, any more than does the mountains in I went to Austria. here's a picture of the mountains in Austria. it is highly unlikely that my one photo shows 'all the mountains' of Austria, nor that that was the meaning of my utterance.
by the way, the resource you cite in your question is full of errors, but second language teachers have to start somewhere.