I've heard the same thing, but I don't give it much credence. I am a manufacturing engineer myself, and I have done both manufacturing support and product design. I earned more in product design, but that was because I was more experienced and more skilled when I did that.
Generally, salaries for engineers in the manufacturing areas are less than those working in design functions. But not "a lot less". There are all of those generic salary surveys on the web to back this up, find them.
A little hint, though: it ain't all about the money. THAT is a fool's game. I've worked in a design function twice early in my career and utterly hated it. For me, it was boring, tedious, heinous work, working "in a box full of testosterone junkies". But I have friends who LOVE that kind of work.
I migrated to the manufacturing engineering side of things and never looked back. I would say that I've chased the skills and topics that interested me the most and now it is quite satisfying, both in content, challenge, and salary. For me, manufacturing engineering is more dynamic, has more variety, and you get to play with the coolest toys. I enjoy it a lot more, and that enjoyment is very much worth the few thousand dollars in salary difference.
For my 30+ year career I have wondered why salaries are less (here in the US). I suspect that it is because manufacturing is not considered a glamorous field. My experience is that manufacturing engineering is a subordinate function to Production Management (getting the parts out the door) and most Production Managers are flaming idiots. So people get disgusted, leave the field, and it becomes populated by 3rd-rate talent. Then also there is the Corporate Management philosophy of slapping an "engineering" title on anybody. I once worked for a guy who had NEVER set foot on a college campus, but he had the title of "Advanced Manufacturing Engineering Manager". He wasn't cut out for the job, was over-promoted, fought desperately to maintain his undeserved position & title, and effectively destroyed the company from the inside-out. This happens very often in this field.