In this context, riffing doesn't just mean improvising - it means to use an existing idea as a starting point for something new.
The term 'riffing' originates in music, but has a slightly different meaning in other contexts.
In music, a 'riff' is a repeated musical phrase, although in many styles of music which involve improvisation (Jazz, for example) it is common to vary the phrase rather than stick to it rigidly. There is also the expression "riffing around a scale", which means to play improvisationally but within the constraints of a number of notes, so more often than not it does resemble a true phrase or motif.
So, in music a 'riff' is a repeated phrase while 'riffing' is to improvise but within set parameters.
Beyond music, 'riffing' means to take an idea and then create new material around it. The idea could be an original one, an idea borrowed from somewhere else, or a 'found' idea such as an observation. It doesn't necessarily involve improvisation, although it can.
For example, a stand-up comedian may make an observation and then write several 'jokes' on that subject. You could then say that he was "riffing on an idea". As those jokes may be scripted before they are performed you would not necessarily say that he was improvising.
In your specific example of a chess game, "riffing" means using a known or documented strategy. Many chess strategies exist, but they cannot be played formulaically because it has to be adjusted to account for the other players moves. It is presented as an idea contrary to "brilliance" which would mean a truly original, genius move. The implication is that if the strategy is known rather than being thought up the spot it is "riffing", or using a variation of an existing strategy rather than creating something new. In this specific context I would have to agree there is some "improvisation", as the strategy is being adjusted on the spot to account for the other player's moves.