But how does a story prove anything about praying mantis?
The story, just literary fiction based on imagined conversations really.

The performance of Han Tong and Zhao Kuangyin meeting is a humorous story that included a skit where Zheng En watched as Han Tong and Kuangyin fought in an inn. As per the custom of those days The lead actor, Kuangyin, sang a song of how he was "beating Han Tong" as both Han Tong and Zheng En interjected between the lyrics of the lead.

The praying mantis sonnet of Taizu is kind enough to mention the techniques used

And, funny thing about Zheng En, he is part of the conversation in the first Chinese erotic novel Jin Pin Mei.
Described by his dark skin and one eye large and one eye small.
And he sits on the sacred alter of several traditional mantis schools!
How does this story weave its way into Beng Bu?