Mandarin and Cantonese share the same writing system in the sense that English and French share the same writing system: mostly the same characters, innumerous cognates, and I can read the back of my cereal box.
Spoken Cantonese is a different language from spoken Mandarin, with different grammar and vocabulary, and when you write them down you get correspondingly different written languages. That written Cantonese hasn't undergone a formal standardization the way Mandarin did in the early 20th century doesn't make written Cantonese gibberish, just not fully standardized. And written Cantonese isn't only used for memeing on forums or text-speak, it's also widely used for casual writing in newspaper columns or in advertisement, or to authentically transcribe spoken Cantonese (rather than paraphrasing in Mandarin). See also:
https://en.wikipedia.org/wiki/Written_Cantonese
https://languagelog.ldc.upenn.edu/nll/index.php?s=written+ca...
[Edit: That kids were (and perhaps are) taught to read and write in Mandarin at school using Cantonese sound values, rather than writing in Cantonese, also doesn't say all that much about Cantonese as a written language. Rather, it demonstrates that Hong Kong is a diglossic (well, polyglossic) society.]
Spoken Cantonese is a different language from spoken Mandarin, with different grammar and vocabulary, and when you write them down you get correspondingly different written languages. That written Cantonese hasn't undergone a formal standardization the way Mandarin did in the early 20th century doesn't make written Cantonese gibberish, just not fully standardized. And written Cantonese isn't only used for memeing on forums or text-speak, it's also widely used for casual writing in newspaper columns or in advertisement, or to authentically transcribe spoken Cantonese (rather than paraphrasing in Mandarin). See also:
https://en.wikipedia.org/wiki/Written_Cantonese
https://languagelog.ldc.upenn.edu/nll/index.php?s=written+ca...
[Edit: That kids were (and perhaps are) taught to read and write in Mandarin at school using Cantonese sound values, rather than writing in Cantonese, also doesn't say all that much about Cantonese as a written language. Rather, it demonstrates that Hong Kong is a diglossic (well, polyglossic) society.]