According to Taiwan’s MOE’s 國語辭典:

「才」與「纔」當副詞時，義並無別，但說文解字：「才，艸木之初也。」因此「才」是本字，「纔」是通假，文獻則多用通假字。

It’s listed under the heading 才. So this use of 才 was around before 纔 came along. And I had thought that 纔 was the more “correct” character, since supposedly it’s used in traditional texts, although I did see that assumption get shot down a bit by a woman from Hong Kong. This makes me wonder about 了解 vs. 瞭解. I may have to check on that later….

Also interesting is this function of 才, which I’ve been coming across recently and has confused me:

表示強調的語氣。如：「我才不幹呢！」、「他哪有累？我才累呢！」

Of course, it doesn’t go into when to use 才 for emphasis as opposed to 就 or anything else, but at least I know about that meaning now.