I am going to say that is mainly because it was newspaper article writing, which is expected to be rather precise by the general public. It is just not written the same way we speak.
Even if 「たち」 had not been used, well over 90% of the readers would have understood it to be plural from the context. 90%, however, is not a good enough number for a newspaper.
"What is gained by saying "学生たち" instead of just "学生"?"
That remaining 10% that I mentioned above. In addition, the publisher only gains if the articles are written in professional manners.
"Can't "学生" be plural without the "たち"?"
Sure can. It is just y'all J-learners who often talk about "singular vs. plural", not us J-speakers.
In my case, I did not even know the words 「[単数]{たんすう}= "singular"」 and 「[複数]{ふくすう}= "plural"」 until I started learning English in junior high school. I am pretty sure most other J-speakers were like that as kids.
"If I were to re-arrange the sentence as such:
デザイン専攻の学生たちが記念誌を作成した。
would it then be ok to replace "学生たち" with just "学生"?"
It would be more than just O.K. if it were another type of writing such as an email to a friend, but in a newspaper article, most people would expect 「たち」 to be there as we have discussed.