Does this lake event in the "Icewind Dale: Rime of the Frostmaiden" adventure allow a grapple as an opportunity attack?

Question

There is an event in Icewind Dale: Rime of the Frostmaiden (an official D&D 5e adventure by Wizards of the Coast) that seems to allow for a grapple attack as an opportunity attack. In the Lake Events table on p. 30, it lists the following result for a roll of 15 on the d20:

A knucklehead trout [...] leaps out of the lake, makes a tail attack against one random character in the boat, and dives back into the frigid water. A character with a passive Wisdom (Perception) score of 13 or higher is not surprised by the fish and can use its reaction to make an opportunity attack against it or try to grapple the fish as it dives back into the water.

To me, this implies that the reaction is used to make an opportunity attack, but that attack can be substituted with the grapple special attack - which goes against the rules as written, which state that a grapple can only be done with the Attack action, not as an opportunity attack.

Now, if the character is before the fish on initiative, it could use the Ready action to ready an attack, then use that attack to grapple. But the text doesn't mention Ready, nor does it state that the fish has lost initiative to the grappling character, which would be required to Ready a grapple. If the fish is earlier on initiative, the character could react with an opportunity attack but could not have an action (to grapple) already Readied.

Does this imply that you can grapple with an opportunity attack, or is it just a case of specific beats general, applying only to this specific fish-slap encounter?

Or is it just alluding to the Ready action, in a roundabout way? (If it is the Ready action, it requires the character was not surprised, rolled higher initiative, and correctly deduced that the fish was going to use a split move to approach, attack, and then retreat.)

Note: This situation does not follow the rules provided earlier for fishing for knucklehead.

Answer 1

These rules are for that encounter, not general.

This is a great example of specific over general! The rules around that encounter for knuckleheads are specific and only relevant for that activity. They don't state anywhere that you can use this option in normal combat or outside of that encounter.

So slap a knucklehead and have some good eats.

Answer 2

A character […] can use its reaction to make an opportunity attack against it or try to grapple the fish

So you can either make an opportunity attack, or grapple. Two separate options.

As NautArch notes, this is also a case of "specific beats general", since the rules don't normally allow you to initiate grapple as a reaction. But even in this case, the grapple isn't an opportunity attack, which could matter in some edge cases (such as having a bonus specifically to opportunity attacks).