Is my surfing level good enough for a boat-trip in Sumatra?

Yes, as long as you are not an absolute beginner.  If you catch waves by yourself (without being pushed) you are ready for a Sumatra surf trip.
Of course some spots are for expert surfers, but that happens in all surfing areas around the world. You don’t have to surf there; actually many surfers don’t.
There are literally hundreds of surfing spots at the Mentawai, Telo, Nias, Hinako and Banyak, and of course there are waves for surfers of all levels.
Our surf-guide pays particular attention to those who are surfing in Sumatra for the first time, advising about each surf spot, how and when to paddle out, the best tide, if you need reef-shoes in that particular spot, what not to do in that wave, etc.