Scratch-cover products vary, but in general they are a very fluid combination stain and finish1. They may be intended to only reduce the visibility of scratches, not repair them, although some are made to do both jobs.
some people say it needs to be reapplied occasionally.
That would seem to indicate that this one specifically is not made to be a drying product, i.e. it's the first type described above.
I can see why the manufacturers might do this to make it easier for the average homeowner to use. But it doesn't make for the ideal product since it will remain permanently wet, meaning any scratches, dings and scrapes it is applied to would be dust magnets.
Can you put a finish on top to prevent that?
In theory you might but safely, no. Any non-drying product could undermine finish put over it, meaning it could peel, bubble or flake off.
It's possible that shellac could successfully seal it off and then allow further finishing with shellac, varnish or lacquer, but no guarantees. And if you're going to be using them anyway why bother with the scratch-cover stuff in the first place?
So to my mind now that things like coloured varnishes and "gel stain" (jellied coloured varnish) are widely available scratch-cover products are obsolete as far as woodworkers need be concerned.
Other options for repairing scratches through darker finish that reveal pale wood are to locally stain the scratches2, then carefully build up finish layer by layer, feathering out and flattening as needed according to the original finish thickness and the nature of the damage. You can also use shellac- or wax-based repair sticks, which are a very old-school method for doing colour repairs but still available and remain a viable way to do the job in some cases.
It's tricky work to get a nearly seamless repair, but possible, and just getting the colour right goes a long way to reducing the visibility. Note: when doing scratch repairs always aim to get an exact colour match or slightly darker, never lighter.
1 I use the word finish in the broadest sense of the word here, shiny stuff, since anything that doesn't dry shouldn't be considered a finish per se.
2 Using touch-up markers, applying stain with a fine pointed brush or by flooding and wiping, or using dry pigments.