It's just now becoming possible. Engineers didn't look at biology before, because they didn't know anything about biology, and lacked tools to manipulate molecular systems. Bioengineering itself is a very new field, and a good portion of the academic bioengineering departments that exist now are less than 5 years old! Bioengineering now is in a similar situation as physics was in the 19th century.

That seems as though it is basically my argument. Biomimetic approaches are challenging and lag behind engineering-based ones by many decades.

I looked at your essay, and don't see that you have any evidence showing that WBE is infeasible, or will take longer to develop than de novo AI.

I don't think WBE is infeasible - but I do think there's evidence that it will take longer. We already have pretty sophisticated engineered machine intelligence - while we can't yet create a WBE of a flatworm. Engineered machine intelligence is widely used in industry; WBE does nothing and doesn't work. Engineered machine intelligence is in the lead, and it is much better funded.

I would argue there's no way to know how long either will take to develop, because we don't even know what the obstacles are really.

If one is simpler than the other, absolute timescales matter little - but IMO, we do have some idea about timescales.