I can tell you with 100% confidence this means they are porting the PS3/X360 version (on the ignite or an older engine) or possibly Wii U version of the game to Switch. They modified an older engine to work on Switch, that's what's "custom" about it. If they used the PS4/X1 version running on frostbite, and customized that for Switch, they'd say it was a "Custom Frostbite Engine". They'd barely be able to contain how they got the PS4/X1 version running on Switch so they could hype how much of a console-like experience you'd be getting on a handheld.

And I don't necessarily blame them or think they're lazy for doing so. Porting console games to compatibly weak hardware is the hardest thing I did making video games. Having an engine that runs well on the system from the get-go makes everyone's life a whole lot easier. That's not to say that the Switch isn't capable of running a modified PS4/X1 version, and that may come in time. I'm just saying this was their best chance of making the most polished game possible for their first foray onto Switch.

Yeah frostbite engine is a bit of a beast and not many ever expected it to run. Switch wouldn't handle it well. good decision not using it.

That's cause they cut support for last gen consoles on their current version ... (The last time Frostbite 3 was used for last gen consoles was Battlefield Hardline ? A two year old game and even Activision's latest Infinity Ward engine which is used for COD games also doesn't work on last gen consoles.) 

Especially this ... 

Frostbite 3 is the newest version, and Battlefield 4 was the first game to use it. Thus, there is a 7th-gen version of the newest generation of the engine, although it's possible that the engine has since been tweaked in a way that doesn't necessarily work too well on 7th-gen consoles. That said, if there were any major differences, I imagine it wouldn't be called Frostbite 3 anymore but Frostbite 4 instead. And that said, it's also possible they've simply migrated from using numbered versions of the engine altogether and simply refer to it as Frostbite, and that the newest versions of the engine simply don't support 7th-gen consoles. It's a bit hard to tell what the situation is right now, unless someone can confirm that they've recently called it Frostbite 3 instead of just Frostbite.

They just didn't want to use frostbite. So they can avoid their faces getting tired, while having bad animations.

If they really wanted, they could have made the frostbite run on the Switch.But that came with two problems: the investment might not have compensated, because the Switch could have been a dud(We didnt know that until about 2 or 3 months ago), and optimizing the engine could mean that they would not be able to release FIFA on time with the other versions, and they could not afford to lose the chance to release a football game, the most popular sport of them all, in a system first year.

If FIFA turns out to be a success, and given the breakout success of the Switch, we may see the Frostbite running on Switch by next year.

But they make a custom game engine. Isn't that more work.

Depends on how easy the engine adapts to the hardware.And lets face it, if Frostbite was easier to port to, why make a custom engine that takes more work, and thus more money?Plus the Switch FIFA game would be an inferior version either way(graphically speaking), so at least with frostbite it would be easier to market the game, because EA could say that it runs in the same engine as the other versions.

So what's their excuse for not including the journey mode in the Switch port. IMO the journey is the best mode in FIFA right now.

I like how EA apparently thinks that gaming has begun with the Playstation and everything that came before doesn't exist.