At the time they flew by it was beginning to diminish, not build.
Satellite infrared imagery returns the temperature of the cloud tops, indicating the height of these tops. Yes, that height was reducing but, IMHO, that does not exclude the possibility that a newer cell was developing underneath.
See also BEA's 2nd Interim Report, para. 1.7.3:
Analysis of the observations by the TRMM TMI instrument, the only one
operating in the microwave area, indicates the presence of strong condensation
around 10,000 metres altitude, lower than the altitude of the cumulonimbus
tops. This strong condensation would correspond to convective towers active
at this altitude, which confirms the strong probability of notable turbulence
within the convective cluster that was crossed by planned flight path of
flight AF447.