I see others has already responded to this about the results.
Could it be that he just likes being at FDJ though? That it is a good environment? There a lot of factors that goes into the decision if you are staying at a team or an employer, instead of going somewhere else simply for money. Now FDJ probably matched other offers, or they compromised enough, so he is being rewarded after a great season and they are happy to keep him. Everybody wins.
Going from a place where you are performing, you are treated good and you are valued is no easy decision. There are so many aspects to consider in life. I wouldnt say that shows "lack of ambition" at all. Especially since he is a leader for the team in a lot of races already. If it aint broke, dont fix it. Grass aint always greener and those sort of sayings.
Ambition can be a great force but is also something that can be insatiable for a lot of people, because most people in reality are just looking for fulfillment. Some are never satisfied, even how far they come and not happy at all because they never feel like they are being fulfilled or have any purpose.
Kung had a clause in his contract with FDJ that allowed them to match any offer he received from other teams - https://www.velonews.com/news/road/stefan-kung-set-to-stay-at-groupama-fdj-as-team-dsm-miss-out/