Haskell concurrency: what if one thread but set + RTS -N

Question

Haskell concurrency: what if one thread but set + RTS -N

As stated in the GHC documentation, the installation +RTS -Nwill use multiple threads to run the program. What if my program is a simple single-threaded program (i.e. No par, no forkIO) as shown below?

m = 10^6

wr 0 = return ()
wr i = do  
        appendFile "/tmp/data.nothing" $ show(i)++"\n"
        wr (i-1)

main = do
    wr m
    putStrLn "done"

However, when I run the program with the flag -Ds(debugging scheduler), I got the following log

all threads:
threads on capability 0:
threads on capability 1:
    thread    7 @ 0x7fd5b0199000 is not blocked (TSO_DIRTY)
    thread 1944 @ 0x7fd592dc6390 is not blocked (TSO_DIRTY)
    thread 1945 @ 0x7fd592def8a8 is not blocked
    thread 1946 @ 0x7fd592dc6790 is not blocked
    thread 1947 @ 0x7fd592e0f790 is not blocked
    thread 1948 @ 0x7fd592df2420 is not blocked
    thread 1949 @ 0x7fd592df2000 is not blocked
    thread 1950 @ 0x7fd592e0f000 is not blocked
    thread 1951 @ 0x7fd592ddae90 is not blocked
    thread 1952 @ 0x7fd5b0156cc0 is not blocked
    thread 1953 @ 0x7fd5b2a2adc8 is not blocked
    thread 1954 @ 0x7fd5b2920568 is not blocked
    thread 1955 @ 0x7fd592c4b5a0 is not blocked
    thread 1956 @ 0x7fd5b289e7a8 is not blocked (TSO_DIRTY)
    thread 1957 @ 0x7fd592c4e160 is not blocked (TSO_DIRTY)
    thread 1958 @ 0x7fd592dc54a8 is not blocked (TSO_DIRTY)