It is at least partially true. French government was informed by Togo that several Belorussians were detained in aftermath of the Bouaké attack. A letter attributed to Togo's then-Minister of the Interior, François Boko, stating that pilots involved in the attack were amongst them was published by Wikileaks in 2010. France then refused to accept extradition of these people, with officials responsible later stating that they did not have enough legal basis to arrest these people.
According to this Slate Afrique article, during 2005 trial, former minister Michèle Alliot-Marie, stated that she was told that there were no legal basis for the extradition; and that seizing the pilots after they landed was deemed too dangerous:
Dans une question, la juge Michon fait remarquer à Michèle
Alliot-Marie que les huit mercenaires biélorusses détenus au Togo,
auraient pu alors être auditionnés par des magistrats ou enquêteurs
venus de France, qui auraient pu ensuite demander leur extradition.
Réponse de la ministre:
«On m’a indiqué qu’il n’y avait pas de base juridique puisque pas de
mandat d’arrêt international. C’était un membre de mon cabinet qui m’a
répondu».
[...]
Pourquoi alors ne pas les avoir interceptés à leur descente d’avion?
«Il s’agissait d’une zone de combats. On ne pouvait pas mettre la vie
de nos soldats en danger, explique Alliot-Marie sans se démonter. Ce
qui n’a pas empêché les militaires français d’effectuer une action
bien plus dangereuse: la destruction à coups de hache des deux
appareils.
To explain the last paragraph of the quote - as a retaliation to the attack, French president Jacques Chirac ordered French forces garrisoned in the airport to open fire on Ivorian combat aircraft there; they damaged several, including the planes which participated in the attack. A skirmish with Ivorian armed forces broke out. It is hard to estimate how realistic the capture of the pilots would be in this circumstances; some sources seem to imply that a the moment of the French reprisal attack they have already dismounted and left the airfield.
Accorrding to the legal adviser to the MoD in 2004, who was questioned in a new trial in 2012, he was not consulted at the time:
Interrogé en 2012, le conseiller juridique en poste en 2004 au ministère de la Défense a déclaré sous serment que non seulement, il n’avait pas été consulté, mais que s’il l’avait été, il n’aurait certainement pas donné cette réponse. En réalité, tout a été fait pour que les mercenaires s’évanouissent dans la nature.
So yeah, it seems to be established that in that particular episode France could've done more to find out the circumstances of the attack.
On the other hand, it seems that Côte d'Ivoire officials denied that any foreign nationals participated in combat. Without their statements, extraditing mercenaries to France would require arresting foreign nationals without sufficient evidence - and according to this le Figaro article, in 2005 it was still not clear who exactly crewed the aircraft involved:
On september 5th 2005, the armed forces' prosecutor gave an opinion opposed to issuing international arrest warrants for two Belarusian nationals suspected of piloting the two Sukhoi 25 aircraft on that particular day. Assistant Prosecutor Ghislain Poissonnier, who was asked about the matter at the end of last week, explains: 'The investigations carried out by the investigating magistrate have not enabled us to formally establish the identity of these pilots at this stage.' Judge Brigitte Raynaud (who has since been replaced by Florence Michon) had come to the opposite conclusion in a ruling dated february 10th 2006.
To sum up: several people were detained by Togo police. Some of them were Belorussian and involved with Ivorian air force in an unclear capacity (not clear if they were technicians or pilots). France then declined their extradition; again, unclear if that was out of genuine fear of triggering an international diplomatic incident, or due to foul play within French government.
