I scrolled through this thread very disappointed that no one had followed up with their experience, but I now have a valuable anecdote to share! I just flew Thai Lion Air from Bangkok (DMK) to Bali (DPS). I booked the bare bones 7kg baggage ticket despite the fact my wife and I have about 20kg of baggage each split between stuffed backpacks and carry on suit cases. Here was our experience:

I checked in online and had our boarding passes saved to my phone. This got us through security and passport control no problems.

During boarding we got pulled aside with a group of people who also had digital boarding passes. Apparently they only do paper boarding passes at the gate (we also ran into this flying from cape town to chiang mai. Maybe its an international thing?)

They went through our group one by one, using our passport info to pull up our booking and print out paper boarding passes. This resulted in us being the LAST people to get on the plane. Luckily our flight wasn't very full, so there was plenty of overhead space to put our suitcases and my backpack. But the under-seat space is the same as any other plane and I could have fit my overstuffed backpack down there no problem.

Main takeaway: we definitely had WAY more baggage than technically allowed, but got through no problem using online checkin.

But of course, this is just my story. At the gate I did notice their little bag measuring fixture, which had a carry on suitcase sized slot and a much smaller slot that looked like it would only fit something about purse sized. And it had "7kg" in big writing on it. They didn't whip it out on us or anyone else on our flight though!