Jacques seems to mean that as a group, lawyers are more open to other domains. This is probably because there is no domain of "the law" by itself--law is merely a reflection of society.
For example, regulatory laws must take into account political restrictions, economic theory and realities, and the goals and pragmatic realities and of the specific industry or industries being regulated. Criminal laws must do all that and take into account various socioeconomic factors. Tax law is a balance of economic theory, revenue generation, political restrictions, and pragmatic realities of accounting across thousands of industries.
That's got a lot to do with it. A lawyer might specialise in some area of law, but the clients who come in will have troubles in or questions about an unlimited number of problem domains. So lawyers get to a surprising amount of the rest of the world outside of their profession.
Some other professions off the top of my head that can't "stick to their knitting": mathematicians, programmers, philosophers, psychologists and economists.
For example, regulatory laws must take into account political restrictions, economic theory and realities, and the goals and pragmatic realities and of the specific industry or industries being regulated. Criminal laws must do all that and take into account various socioeconomic factors. Tax law is a balance of economic theory, revenue generation, political restrictions, and pragmatic realities of accounting across thousands of industries.