The ruler calibration could be off a bit, the precise position of the wires is not easy to measure, the ruler bends a bit. +- 0.5 cm is a conservative estimate.
It doesn't really matter for the experiment he wants to perform.

He demonstrated the measurement improperly. The length has two measures, one at each end. He aligned the "0" of the ruler at one end of the length and assumed 0 error for the measurement at that end, then read the other end with an estimate of error...

The proper way to measure like that is to lay the measuring ruler randomly (not aligning the "0" on the ruler at one end of the length to be measured), take a reading from the ruler at each end of the length, and subtract the greater from the smaller.

In popular culture people set the "0" end of the ruler or tape measure and simply read the other end as their result. Length measures in science must not be done like that. There is a proper methodology for using a ruler, as well as methodologies for estimating error, significant digits, and rounding... and how to take all this into account when doing the math.

He has a 2% uncertainty in the time measurement. Measuring the length more precise than 0.5% is unnecessary.

The dominant uncertainty of the length measurement is not the reading of the ruler either, it is the 3D shape of the ruler and possibly its calibration.