There is a major dispute in the mediaeval Rabbinic authorities about how to understand that Gemara. (see the Biur Halacha on Oroch Chaim siman 530) Some understand that the work on Chol Hamoed is rabbincly prohibited with this Gemora being an asmachta. According to that they probably did work until the prohibition against it was enacted. Unless Moshe Rabbainu himself first added this rabbinical prohibition.
Others understand it to be saying that work on Chol Hamoed is Biblically prohibited . According to that there had to have been some work prohibitions on Chol Hamoed .
The concept of "Lo mosron Hakosov eleh l'chachomim" (that the Torah left it up to Chazal to figure out the exact parameter) comes up elsewhere too. The Chazon Ish (Yodoyim 8:17) explains the concept in a general sense that there frequently are competing calculations of the correct course of action (in halachic terms "mi nidcha mpnei mi")the Torah is instructing the Rabbis to decide on the proper one. (i.e. which work prohibition would or would not have other considerations causing them (not) to be prohibited.)
Possibility in the earlier generations, which work prohibitions were (not) avoided on Chol Hamoed was decided by the Rabbis of that generation depending on the situations they were facing. Afterwards there came a time when a universal Rabbinic consensus made the rules permanent and immutable.
The later authorities bring the frequently quoted Gemora in Yerushlmi where Rav Abba Bar Mommal said that if he could find others who agree with him he would allow some of the prohibited work on Chol Hamoed as an indication that there was a POSSIBLE fluid nature of the work prohibition on chol H'Moed at that time. Even if a Beis Din that was bigger in number and wisdom had already prohibited that particular work prohibiton (they don't consider this proof to be 100% conclusive)
