What does the Milelion invest in? Do you think luxury travel hacking would significantly impact savings for retirement?

I invest mostly in US equities, with a couple of index funds as well. Apart from that, I always max out my SRS contribution and CPF SA top up for the year.

The whole point of travel hacking is that you don't spend big when you travel, so I don't see why it should impact savings. Perhaps to the extent that you're able to take more trips, so you end up spending more because of more vacations? That comes down to individual self-control, however. Travel hacking just makes the hotels and flights cheaper- what you do when you're there is up to you!