Is it correct for the Imam to say that all the worshiper's takbirat are to be said silently except for the takbirat al-ihram and the taslim, as a sign of khushu' (devotional humility)?

Scholars have differed regarding the congregant (other than the Imam) vocalizing the Takbir (Allahu Akbar) and Taslim (Assalamu Alaikum). Many of them disliked it, preferring only the Imam to vocalize, unless there is a need, in which case it is not disliked.

However, the Malikis hold that the congregant vocalizes only the opening Takbir (Takbirat al-Ihram) and the Taslim of concluding the prayer (Taslimat al-Tahlil), and recites silently in all other instances.

The Malikis justified the congregant vocalizing the Taslimat al-Tahlil by stating that it is sunnah for congregants to return greetings to one another, so it is vocalized for their companion to hear it and return the greeting.