The absence of specifying when something happens in Chinese
In Chinese sometimes a sentense may lack a time, which is quite different from English, which has a specific tense. In this case the tense depends on the text and the language environment but not always means a same way. 

For example, imagine that if you were buying a ticket and saw some people who hold tickets, "他们都拿着票" would mean progressive tense. If it is a narrative, it may means past tense.

As for "了"，"了" can be used in many tense.
See this Tense and use of 了