In what way?

Shenzhen and HK are both dominated by same race of people, culture, religion and language.

Is Johor and Singapore similar? It's so different.
It is only true if HK was majority owned by Ang Mohs/Indians and Shenzhen is majority Chinese then you can say is comparable with Johor/Singapore. But it's entirely different.

Also Guangdong has another rich neighboring country which is among the richest in Asia per capita besides HK. It's Macao SAR.

Johor got 2nd neighboring country attached to it?

Both HK and Macao are SARs which the motherland has somewhat control over them over their economic roles unlike SG/Malaysia which are direct competitors.