Although he did specifically call out Ruby on Rails I rather think he meant ActiveRecord the pattern [0] rather than ActiveRecord the implementation [1]. Ruby on Rails' ActiveRecord has come a long way and actually implements proper relational algebra through Arel [2] now.

[0] http://martinfowler.com/eaaCatalog/activeRecord.html

[1] http://ar.rubyonrails.org/classes/ActiveRecord/Base.html

[2] https://github.com/nkallen/arel