I think this explains it. From Wikipédia:
https://fr.wikipedia.org/wiki/94e_r%...#Second_Empire, thus about 1970/1.
Quote:
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À partir du 4e bataillon, le 94e de ligne est reformé au sein de l'armée Chanzy avec le 1er régiment des grenadiers de la Garde impériale et des prisonniers libérés. Il prend depuis cette époque le nom de régiment de la "Garde".
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Or:
Taking the 4th battailon as a starting point, the 94th line was reformed as part of the Chanzy army with the 1st regiment grenadiers of the imperial guard (of Napoleon III, who abdicated) and freed prisoners. Since this time it took the name regiment of the Guard.
Thus it seems to be part of their own traditions, but not very official.