Ah. It is best to explain our actions.
It was our intention to "triple-cross". We pretend to betray you, whilst actually betraying Vegan. It was an attempt to gather intelligence on the NPTO headquarters, and on Vegan actions, along with a shot at actually assassinating her. We purposely removed an encryption on the letter to make it more believable; with an "accidental encryption fail", then you wouldn't trust us, meaning that Vegan would take advantage of your shock/disbelief and verify that we had in fact "betrayed" you. We didn't inform anyone because we couldn't spare a single slip of the tongue which would undermine our plan.
However, this plan failed when May Leng re-couped Guslantis. Since Vegan was no longer is control, then there is no point attempting to gain her trust; she is worthless for intelligence purposes. However, we do have a small fleet of men on Guslantis' shores, who were ready to infiltrate the area to gain intel.
We are very sorry for any shock caused to people, and we also do not think in any way that you are imperialist; far from it, seeing as you are not conquering countries in the name of expansion. That was merely an additional adjective to make it seem like we had distaste for you, of which we do not. The operation to liberate the NPTO HQ is still on, however.
In short, we are sorry for "betraying" you. You could not know about it because then Vegan would suspect your fake disbelief and uncover the plot, or you could have accidentally slipped an accidental word out. This was to gather intelligence on Vegan and the NPTO HQ, to conduct a more successful raid in which a liberation chance would be more likely with a lower amount of casualties, and to also attempt to assassinate Vegan to bring an end to the Empire.