I think you can find the proof of that statement in most Playbills.
Anecdotally, students at D’s school (Northwestern) had a wide range of experiences in getting representation. Quite a few kids arrived on campus with strong representation which they’ve kept, some got agents during their 4 years of college through exposure during professional shows, some were cast in major tours or even Broadway and opted not to get representation (they didn’t want to give up a % of their salary since they were already employed), some chose to skip Showcases as they had conflicts with existing contracts (this happened to D) and many took agents after showcase. All of the kids I know of personally in D’s MT cohort (they graduated in June) who want representation have gotten it by now. My anecdotal experience might indicate that a Showcase isn’t needed for talented kids to land great representation, but that it can certainly help.
As @soozievt says, it is not uncommon for kids to switch agents as their careers progress, and it is important to understand that all agents are not created equal. I know kids who are rarely submitted by their agencies and others who have to turn down many submissions as they’re too busy.
I’ve also heard that college Showcases are becoming obsolete for many reasons but don’t personally have the expertise to say whether or not that’s true.
Best of luck in your college search!!