I'm wondering why this thing is called 'antimatter' reactor while it uses singularity cells. Is this a singularity reactor?

In my game it's using Charged Antimatter cells, that you get once you unlock the late game matter conversion stuff... Maybe there's some other mod in your list affecting those?

No, There are no mods that could affect this. Moreover, I see the description of a singularity reactor (not portable) and it says the reactor consumes... antimatter fuel! However, there is no way to insert antimatter cells in it, just singularity ones, so it works as expected.
See here: https://imgur.com/a/CiT4O86

I guess I figured that out. This happens when SE mod is also activated, sorry for confusion. For SE compatibility, some fuel types have been changed:

if mods["space-exploration"] and krastorio.general.isVersionGreaterEqualThan(mods["space-exploration"], "0.3.0") then
-- Chaning the name of the antimatter reactor fuel for not be the same of the antimatter reactor of SE
data.raw.item["empty-antimatter-fuel-cell"].localised_name = { "item-name.empty-singularity-fuel-cell" }
data.raw.item["charged-antimatter-fuel-cell"].localised_name = { "item-name.charged-singularity-fuel-cell" }

I'm not sure why it was changed like that, but it is REALLY confusing