Not only were they starved, they were starved for a lot longer than the 6-day duration of the mission. According to this translation of Gaidamakin et al. (1969), the "turtles" (steppe tortoises, Testudo horsfieldi Gray) were actually starved for 39 days:
The turtles were put on board the "Zond-5" on 2 September 1968. From that time on they ceased to received food ... lift-off was on 15 September ... splashed down on 21 September ... returned to Moscow on 7 October ... Patho-morphological investigations [i.e. dissection!] on the turtles were carried out on 11 October.
As is seen from the cited sequence ... the test animals were subjected to 39 days of starvation, flight factors lasting 7 days, the effect of tropical climate and conditions associated with a stay in the ocean after splashdown, and with transportation via ship and aircraft.
Weight loss of the animals that flew in the probe was about 10%. Weight loss of the control animals was only 5%.
Gaidamakin, N.A., G. P. Parfenov, V. G Petrukhin, V. V. Antipov, P. P. Saksonov, and A. V. Smirnova. “Patho-Morphological and Histochemical Changes in the Organs of Turtles on Board the ‘Zond-5’ Probe.” edited by George R. Zug and James A. Peters, translated by Morris D. Friedman. La Plata, Argentina: Smithsonian Herpetological Information Services, 1969. https://repository.si.edu/bitstream/handle/10088/32682/1970.SHIS24.pdf.