I actually think that because of the quirks of Clan society, it would have been even more difficult to hide than if they had been Inner Sphere powers.
The structure of the Clans is that the entire Clan is supporting the warrior caste. Now I know that's not literally true, but it is true that a lot of the lower caste have jobs that are directly or indirectly tied to the warriors. With their budgets and priorities established by the warrior caste and the Clan Council. Even a scientist caste researcher in the Homeworlds, specializing in genetically engineering new crops to yield more food, might see their budget allocation changed because the Clan's priorities changed because of the ramp-up to Tukayyid (or Op. Revival in general) or due to the resource change needs of the aftermath (needing to rebuild the touman after such heavy losses).
So if the warriors need to prepare for a major operation, or recover from the aftermath of one. There are a lot of lower castes involved in that, and not just those attached to the touman directly as support personnel. In the aftermath certainly, the lower castes would have noticed a shift in priorities as Operation Revival ended and priorities shifted.
A great example is the Shark merchant caste. In the aftermath of Tukayyid it's well-demonstrated in canon that they were given a lot of latitude with orders to rebuild the touman. They ran with that and spread far and wide, driving hard bargains everywhere. Odds are, the reasons why they were doing that developed into common knowledge.
So no, I don't think it was a secret. However what was said (in official statements anyway) was almost certainly laden with lots of pro-Clan, anti-Inner Sphere propaganda. Talking up how dishonorable their barbarian opponents are, and how this demonstrates what some of the Crusaders have been saying is true, that they are indeed a threat. Behind that was a lot of more honest information that probably spread more person-to-person over a longer period of time.
The Crusaders' mantras had two, arguably contradictory central pillars to them. One of them was that the Inner Sphere was a serious military threat and the Clans needed to attack before the Inner Sphere attacked them. They pointed to the Explorer Corps Jumpship discovering Huntress as justification for that line of reasoning. The other argument is that the barbarians are weak, ruled by unsavory selfish warlords (the Great Houses) interested only in advancing their own selfish needs, and that in the Succession Wars they had decimated their own nations and killed many and reduced beautiful worlds to wastelands, and that the Clans were actually doing the people of the Inner Sphere a favor by conquering the Inner Sphere and bringing the Clan superior society to the barbarians. That it was akin to a humanitarian mission really, saving the Inner Sphere from the real villains.
I'm sure in this there were a lot of comparisons to Operation Klondike.
After Tukayyid, parts of the despotic barbarian argument don't look so sharp anymore. The Inner Sphere doesn't look entirely weak and the poor citizens of the Inner Sphere that Clan propaganda claimed they were saving are actually fighting back against the Clans. It muddies the water of the second Crusader argument. But the first argument, that the Inner Sphere is actually a threat to the Clans. That line of propaganda looks as good as ever. The Inner Sphere is actually a threat, they can defeat us, so we need to be stronger than ever before. The Crusaders can hang their hats on that one post-Tukayyid as they dole out the propaganda to their lower castes.
