If a Chuul grapples a creature as a result of its Multiattack, can it make a tentacle attack on the same turn?

One of my player’s and I differed on how we interpreted the Chuul’s multiattack the other night. Below is the MM text:

Multiattack. The chuul makes two pincer attacks. If the chuul is grappling a creature, the chuul can also use its tentacles once.

The situation was as follows:

The chuul attacked player A with its first pincer and missed. It attacked A with its second pincer and hit, thus grappling A. I ruled that the tentacle attack could occur on the chuul’s next turn if A were still grappled. The other possible ruling is that the tentacle attack can now occur on the chuul’s current turn.

My thinking is that a Multiattack is one action, and that the “If the chuul is grappling…” is a prerequisite condition to the action that must be met to grant the tentacle attack. The wording of “grappling” as opposed to “grapples” means that the chuul must be currently grappling the target before the Multiattack action is taken.

Is my interpretation correct?