Compare the following from Cantonese Wikipedia:
粵文維基百科 係維基百科協作計劃嘅粵文版，由非牟利組織──維基媒體基金會喺2006年3月25號成立。
To "mandarin" Wikipedia
粵文維基百科是維基百科協作計劃的粵文版本，於2006年3月25日成立，由非營利組織──維基媒體基金會負責營運。
1. Save for 係, 嘅, and 喺, the former is perfectly valid Mandarin. Every other difference between the two texts are phrasing differences from then being written seperately by different people. If you did s/係/是/, s/喺/於/, and s/嘅/的/ and copy pasted the Cantonese version into regular Chinese Wikipedia nobody would notice.
2. Despite the fact that not a single word in those two excerpts would be pronounced the same, you can clearly see the same characters being used even if you can't read Chinese. This can not be the case if it were a "syllabary".
3. 係, 喺, and 嘅 are written differently not because they're pronounced different(so is every other character in the above excerpt) or even because they have a different meaning(they pretty much serve the same grammatical purpose as 是, 於, and 的), but because they have different etymologies and are different morphemes. This can only be the case if the writing system is logographic. It wouldn't be the case in either a logographic or ideographic system.
Mind you Cantonese Wikipedia is written exclusively by people like you who think Cantonese is a "language", and it's not some dying dialect(It's the second largest dialect and probably spoken by at least 50% of Chinese speakers who actually use Wikipedia and not some other wiki) like Scots so you can't claim that the people writing are just bad Cantonese speakers.