ECM is Electronic Countermeasures. ECCM is Electronic Counter-Countermeasures. The former attempts to prevent enemies from utilizing the electromagnetic spectrum, the latter attempts to defeat enemy ECM.
There is no such thing as a guidance system immune to ECM. Please do not accept the manufacturers brochure as being a fair and impartial analysis of a weapons system...
Perhaps the Yahont carries its own ECM, thus the need for data on the adversary's (US) ECCM systems on board? What the description said was that the missile carries ECM and ECCM data in its memory, as well as the means to evade "the enemy's air defense systems". I can't see why it'd need ECCM data if it didn't have its own active ECM.