An antonym for "shortcut"
| It's not a shortcut, it's circuitous. → adjective, paraphrase
| It's not a shortcut, it's a diversion. → not a direct path
| It's not a shortcut, it's a long shot. → not a safe bet, two words
| It's not a shortcut, it's a dead link. → bad web bookmark, two words
Hellion's suggestion of detour is an excellent choice as well, although his other suggestion scenic route appears to be the popular choice. I don't think scenic route is a good choice for the sample sentence, it seems overly redundant. But it works great in other contexts, like: I'm enjoying our conversation, so let's take the scenic route.
I believe in the context of your use case, you can use circuitous, or one of its synonyms (which include indirect, circular, or roundabout), as in:
| It's not a shortcut, it's circuitous.
: not straight, short, and direct
Merriam-Webster online
For a noun word, you can use diversion.
The connotation is not quite opposite of shortcut, but it will imply the path is not direct.
: the act of changing the direction or use of something : the act of diverting something
Merriam-Webster online
When the word shortcut is used to indicate a relatively safe path, as in "a shortcut to success", a word with a nice ring for opposing it would be long shot.
: an attempt or effort that is not likely to be successful
Merriam-Webster online
Finally, in the use of shortcut in the context of a web browser bookmark, the only opposing sense I can think of is a dead link.
A dead link is a link on the world wide web that points to a webpage or server that is permanently unavailable.
Urban Dictionary
Well, I have further reflected upon stale link (the link no longer points to what it originally pointed to). A little more whimsical would be to use the word breadcrumb:
| It's not a shortcut, it's a breadcrumb.
This would imply the web link merely takes you to a starting point to find the true thing of interest, rather than taking you directly to the thing of interest. However, it is kind of an abuse of the term, since in computer jargon, it refers to the collection of "back links" that the browser tracks for you so that you can back your way out to your starting point.
Depending on the context, "Scenic route" could serve as an antonym.
I don’t believe there is a single word in common use. The most common term I know of is long way or long way around.
I have heard (and used) longcut in jest. Wiktionary indicates that longcut may be in use, but there is a divide as to how it’s used: it indicates either the opposite of a shortcut, or a route that was supposed to be a shortcut but in fact turned out to be longer. Urban Dictionary corroborates this.
The thing about the word "shortcut" is that the word is positive. "We don't have to go that way, there is a shortcut" There is no similar opposite word because to remain positive one would need to understand why the other way is chosen.
So we have two possible opposites. If the other way just wastes time, it is referred to as "roundabout". This implies that any way but the shortcut way is not perfect, so you would have the one shortcut way and all the other roundabout ways.
The other word is "proper" This means that there is something wrong with the shortcut, and there is a benefit to the other way. "Using the calculator is a shortcut, do the math problems the proper way"
I would say: "It is not a shortcut, it is a detour."
Detour - Noun
A long or roundabout route taken to avoid something or to visit somewhere along the way
Verb, Chiefly North American
A long or roundabout route taken to avoid something or to visit somewhere along the way.
Origin:
Mid 18th century (as a noun): from French détour 'change of direction', from détourner 'turn away'