The file linked is not what the 'simple Haskell' philosophy suggests. It is compiled with the following extensions enabled:

    OverloadedStrings
    NoImplicitPrelude
    ImplicitParams
    Rank2Types
    DisambiguateRecordFields
    NamedFieldPuns
    DuplicateRecordFields
    OverloadedLabels
    FlexibleContexts
    TypeSynonymInstances
    FlexibleInstances
    QuasiQuotes
    TypeFamilies
    PackageImports
    ScopedTypeVariables
    RecordWildCards
    TypeApplications
    DataKinds
    InstanceSigs
    DeriveGeneric
    MultiParamTypeClasses
    TypeOperators
    DeriveDataTypeable
    MultiWayIf
    UndecidableInstances
    BlockArguments
    PartialTypeSignatures
    LambdaCase
    DefaultSignatures
    EmptyDataDeriving
    BangPatterns
    BlockArguments
    MultiWayIf
    FunctionalDependencies
    PartialTypeSignatures
    StandaloneDeriving
    DerivingVia

Most of those feature in this list of recommended defaults https://github.com/commercialhaskell/rio#language-extensions which is cited as an example in simplehaskell's page on recommendations. https://www.simplehaskell.org/language-extensions.html

From what I've touched of Haskell, I'd think e.g. OverloadedStrings makes Haskell simpler to use than leaving it out. (And OverloadedStrings certianly doesn't seem ego-inducing).

Simple Haskell's page on extensions also isn't advocating for anything too concrete. Just an emphasis on pragmatic, simple to maintain code.

IHP's landing page even claims "Don't worry, you don't need to know much about Haskell to get started, you'll learn it along the way". This seems to be in the spirit of simple Haskell, although I'd be curious to hear about what parts seem not-simple.

The extensions that aren't on that list but are in the list provided by lalaithion are pretty "big" extensions that can severely increase the bar needed to understand Haskell code (I'm looking at ImplicitParams, QuasiQuotes, DataKinds, TypeOperators, and UndeciableInstances).

In general, I've only read over IHP code, but never used it myself, I'd be worried about the quality of GHC error messages for a Haskell novice with the amount of magic IHP is providing.

There's some nifty type-level programming going on in the background with IHP which works great when everything compiles, but can have confusing GHC error messages if you have an error.