John Buchan reference - true life?

Factual Questions
1

In the book Greenmantle by John Buchan (he wrote The 39 Steps), one character (Peter Pienaar) is described as follows: “He was in Swaziland with Bob Macnab, and you know what that means.”

Is this a true life reference?
Or was Buchan just assuming his readers in 1916 would understand that sort of reference?

2

If you’re asking whether Bob McNab was a real character, it seems the answer is yes. Here’s one cite, and here’s another.