SegWit wasn't fully activated until yesterday, so that time you payed the lower fee was during a time of less transaction traffic.
Different wallets will have different roadmaps on when they are ready to support SegWit. I know that SatoshiLabs are planning to add Bitcoin SegWit support to their Trezor beta-wallet in a few days, so I guess other wallet providers won't be far behind. The way I understood it you'll have to move your BTC to a SegWit enabled address first (starting with a "3", like a multi-sig address) before being able to send a SegWit transaction, so that part of the process will probably be manual and require action on your side.
If a majority of the transactions we see at the moment are spam, and segwit needs to be activated by the user, how will segwit help to decrease the mempool?
The guys who make the spam transactions will simply not activate segwit.
Or have I missed something?
There's rumour that since that BTC has been under a spam transaction attack since at least the beginning of the year (ie. since the scaling debate started to get serious). Thus making the network seem more over capacity than it actually is. One thing that speaks for this rumor is that the spam attacks subseded late Juli when SegWit (or was it Segwit2x?) locked in.