What’s the meaning of ‘one thing’ in sentences like “Jack is one thing, ...

The context is a conversation between a teacher and two of his former students. The teacher asked about the current jobs of his students (the students’ names are Jack and John), and they answered:

1. We’re now running a small company.

Then the teacher said:

2. Jack is one thing, but I’m surprised that John can do business!

What is the meaning of one thing in the sentence above?

Can sentence (2) be rephrased like this sentence (3)?

3. I’m surprised about Jack, but I’m even more surprised about John’s business ability!

From the OED:

thing noun¹
Phrases
P.2.b. one thing: something acceptable or satisfactory (or at least not as bad), contrasted with another (thing) that is unacceptable or unsatisfactory (or even worse). Also: something fairly easy contrasted with another (thing) that is much harder. Frequently in it is one thing (to do something), and another (to do something else). 1587–
_{[selected attestation]
1994   Bad pitching is one thing. Pitiful pitching is quite another. —Chicago Tribune 15 April iv. 3/1}

_{Source: Oxford English Dictionary (login required)}

Your example—Jack [doing business] is one thing but I'm suprised that John can do business!—compares Jack doing business (not surprising) and John doing business (surprising):

It is one thing that Jack can run a business but another that John can!