Sure, I agree, but there are likely standard approaches, especially in the older trades, where the development is making ~1 decision and then measuring.
Like if the carpenter is installing some shelves, they are most likely picking a shelving system or approach they know how to work with and measuring for fit, not coming up with a brand new way to mount shelves. They might come up with a new way, it just isn't all that likely.
The same can be said for a lot of software developers. How many people spend their days gluing together existing libraries vs. writing their own? This rule seems ridiculous; I fail to understand why they're are different tax write off rates for employees based on what they're doing. Either way they're being paid by the business!
The entirety of the rule seems to exist to punish small players in the market. A barrier to entry to box out competition.
The difference is repetition. Many welders work some form of assembly line, where they constantly are welding the same bracket on and sending the part down the line.
Even if they are not on a line, few welders are designing the bracket, instead they cut it out according to blueprints (this might be a separate person) and then weld it on. Then they look at the next part of the blueprint and put it on.
If you have a clear, standard approach to the installation, then the development is likely to be a small portion of the work.
If neither of those are true then you are in the hard to figure area.