It also says "first language speakers" in the overview circle towards upper left hand corner of the whole infographic, so maybe that's what they counted, not native or fluent speakers. That could perhaps explain the perceived low number of Ukrainian speakers of Russian?
also cantonese and mandarin people cannot understand each other, despite sharing some written characters, so lumping them together seems inaccurate, like lumping spanish and italians together because they have the same script.
Cantonese is a dialect of Yuht Yúh (粵語), which is, as you say, a distinct language as it is mutually unintelligible with other varieties of Chinese. Though Cantonese speakers usually write in Standard Chinese (i.e. the written form of Mandarin), Yuht Yúh also has its own distinct written form (https://zh-yue.wikipedia.org/wiki/%E7%B2%B5%E8%AA%9E), as it often uses different words from Mandarin.
It's interesting to compare with the Italic languages (Italian, Spanish, Catalan, Portuguese, French, and Romanian). These are similarly far apart as the Chinese 'dialects'. The Chinese situation is as if the Roman Empire never collapsed, and the Italic language speakers continued to use Classical Latin as a written language, then about 100 years ago decided to replace Latin with Spanish as the written standard throughout the Empire, as it was the most widely spoken language.