Luxon DateTime not recognized as Luxon DateTime object

I am not understanding how Luxon works…
I am using Redux Toolkit. I initialize the state like this

import {DateTime} from "luxon"

interface TestStateProps {
    startDate: DateTime,
    endDate: DateTime,
}

const initialTestState: TestStateProps {
    startDate: DateTime.now(), 
    endDate: DateTime.now(),
}

export const testSlice = createSlice({
    name: 'test',
    initialTestState,
    reducers: {
        addDay: (state, action: PayloadAction<number>) => {
            console.log(state.startDate instanceof DateTime)
            console.log(JSON.stringify(state.startDate))
            console.log(state.startDate.plus({days: action.payload }))
        }
    }
})

And now, the result of the console.log is quite unexpected

1. console.log(state.startDate instanceof DateTime)

false

Why is it false? I initialized it with DateTime.now()

2. console.log(JSON.stringify(state.startDate))

{“ts”:1724911978667,”_zone”:{},”loc”:{“locale”:”en-DE”,”numberingSystem”:null,”outputCalendar”:null,”weekSettings”:null,”intl”:”en-DE”,”weekdaysCache”:{“format”:{},”standalone”:{}},”monthsCache”:{“format”:{},”standalone”:{}},”meridiemCache”:null,”eraCache”:{},”specifiedLocale”:null,”fastNumbersCached”:null},”invalid”:null,”weekData”:null,”localWeekData”:null,”c”:{“year”:2024,”month”:8,”day”:29,”hour”:8,”minute”:12,”second”:58,”millisecond”:667},”o”:120,”isLuxonDateTime”:true}

I guess this is expected "isLuxonDateTime":true. Why is state.startDate instanceof DateTime returning false? When I check in the redux dev tools, its not deserialzed or something

4. console.log(state.startDate.plus({days: action.payload })) finally throws an error

Uncaught TypeError: state.startDate.plus is not a function

What am I doing wrong here?