A sidechain is simply an alternate/secondary chain that shares a two-way link/peg with another chain that is the primary chain (frankly primary/secondary are philosophical roles that are interchangeable per case), to exchange messages and assets.
With that said, creating a variant of Cardano via a fork of the canonical chain from a point or a whole new chain/genesis block from the original codebase (with changes), doesn't automatically make that variant a sidechain of Cardano or inherit the security of the original Cardano network.
This is because
A) bridge and relevant cross-chain protocols need to be deployed for interoperability as a sidechain.
B)security on the canonical chain is weighted in ADA stake which is tied to real world value and largely recognised by global actors. The variant will simply have a bunch of ADA tokens that do not in anyway reflect value of ADA from the canonical network. It will also lack the integrations and network effect of users and SPOs to solidify its place.
So a Sidechain is not a forked chain by default, and vice versa, but arrangements can be made to make a forked chain become a sidechain.
P.S. The whole point of Sidechains, starting from Bitcoin was about building alternate networks that communicated with the original network but had the flexibility to make/follow its own custom rules. This is the idea for sidechains on Cardano, where parties can build their own sidechains with their own rules should they feel misaligned for some reason or experimental.