If someone shows as 1m with no green dot on facebook messenger, have they blocked me seeing when they're online?

On my messenger app on my iphone, if i open a chat to the person, it'll say "active now" but on facebook.com it doesn't show a green dot. Just says 1m.

No they have not Blocked you the Last Active Time Stamp is not always Accurate so if its saying a lot that they been Active 1 minute ago it just has not Accurately Updated

No, it means last active 1m ago.