Did you really just call somebody replying to you in English a "monolingual mandarin speaker". There have plenty of times where I have accidentally gone on Cantonese wikipedia and didn't notice until halfway through the article.
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.
Comparing Wikipedia versions is not terribly useful, since formal written Chinese tends towards uniformity: previously 文言, now 普通话.
Spoken Chinese does not, and an accurate transcription of colloquial Cantonese is quite different from colloquial Mandarin: 係唔係佢哋嘅 vs 是不是他們的, and that's despite the grammar of the two sentences being identical (which is not normally the case either).
I was arguing against OP's case claim that Chinese is a "syllabary".
But even then your example is far more cherry picked than mine. Translated your phrase means "is it not theirs". It doesn't have any common nouns that aren't pronouns, adjectives, or even a verb that isn't is/是/係. You also managed to do something I couldn't even and provide an example where every word had the same exact grammatical meaning and were arranged in the same order in both examples. Really helps my point about logography more. If Chinese were ideographic and not logographic those words would be written the same. They're not because of differing etymologies.
The fact that you can only find colloquial examples that are significantly different is why it's a dialect. AAVE or Appalachian English are also near incomprehensible when spoken/written colloquially. Hell, I could even make the dumb arguement that liberals/conservatives speak a seperate language with something like your example. "was he Latino"/"were they latinx?"
Here I found the following sentence on a Cantonese bbs.
"推普滅粵係中國政府堅定不移的國策"
Perfectly valid Mandarin except for 係/是. You can't claim that a bbs comment complaining about the promotion of Mandarin, written on a forum dedicated to Cantonese of all things is formal language that's "tending towards" "mandarin".
That is what is so amazing about monolingual Mandarin speakers believing they should be.