We're confusing equal length and "tuned" exhaust.
Equal length is just as described; all the runners are equal length. For practical purposes, the LeMans exhaust is equal length as the runners are with 4% of each other. In essentially straight pipe, this length difference creates negligble difference in flow resistance.
Tuned exhaust was the holy grail of engine builders. The idea was to built exhaust gas velocity and momentum down stream to help scavenge the cylinders. If it worked at all, it was only good at the tuned engine speed. Lower speeds did not produce the scavenging effect and higher speeds just produced more flow resistance.
The period racing exhaust could more accurately be decribed as "practically equal length, possibly tuned LeMans exhaust"