why would it be hard to find the ideal co2 level if the light intensity were very low?

Because at very low light intensity, photosynthesis is barely happening, changing CO₂ levels will not noticeably change the rate, so there is no clear “best” CO₂ level to detect.

What limits photosynthesis here

At low light, photosynthesis is limited mainly by light , not by CO₂.

Even if CO₂ is increased or decreased, the overall rate changes only slightly because the light-dependent reactions cannot run much faster when light is scarce.

Why “ideal CO₂” is hard to see

To find an “ideal” CO₂ level experimentally, you look for a CO₂ concentration where photosynthesis (or oxygen production) is maximized and then plateaus.

Under very low light, the curve of rate vs. CO₂ is almost flat, so measurements at different CO₂ levels look very similar and the optimum is practically invisible.

Role of respiration

In low light, plants still respire, using O₂ and releasing CO₂, while photosynthesis is weak.

This background CO₂ production can mask small changes in CO₂ uptake by photosynthesis, further blurring any “ideal” CO₂ point.

TL;DR: With very low light, photosynthesis hardly responds to CO₂, so the data do not show a clear maximum; that is why finding an ideal CO₂ level is difficult.

Information gathered from public forums or data available on the internet and portrayed here.