The tldr is that they don't know it's alphabetic for sure (see below quote). The main scholar (Glenn Schwartz) who co-oversaw the '94-'10 excavation isn't an expert in writing. He put it out there around 2010 and said "maybe it's alphabetic, idk" and there was not much followup from the community. So he consulted with some writing experts who helped him with the 2021 paper where he goes over the evidence for different possibilities and suggests that the strongest argument is for alphabetic. The dating seems to be on firmer ground but the error bands on this and Wadi el-Hol can probably knock a century or two off the "500 years".

A decent summary is the blog post below from another researcher who briefly was part of the same dig and a former student of Schwartz (so not entirely independent):

http://www.rollstonepigraphy.com/?p=921

It is worth noting that in the past Schwartz has been reluctant to affirm that the four inscribed clay cylinders from Tomb 4 of Umm el-Marra are alphabetic (Schwartz 2010). Thus, he certainly did not rush to this conclusion. Moreover, his most recent article about these is also very cautious (Schwartz 2021), as he moves through various possibilities (as discussed above). But it is clear that he is now willing to state that this is the most reasonable position (i.e., it is Early Alphabetic). And I concur. That is, the most reasonable conclusion is that the Umm el-Marra clay cylinders are inscribed with signs that are most readily understood as Early Alphabetic letters (graphemes). Moreover, since the Early Alphabetic alphabet was used to write Semitic, it is logical to conclude that this is the language of the Umm el-Marra inscriptions (the fact that they were found in Syria would also augment this conclusion, of course).

The full blog post is worth reading and summarizes the case for various non-alphabetic possibilities.