that looks like a good deal compared to the goalzero 400 lithium (cost 700 dollars), they are both in the 40ah range. The goalzero uses li-ion cells (cylindrical laptop 18650) the flexopower uses lithium polymer which are flat cells (like cell phone batteries but larger). They both are fully charge at 12.6 volts and 10 volts at 0 percent. It will give you usable power all the way to 10 volts then the bms will cut the output.
I don't think you can build your own and make it as small as the flexopower, I built a small 27ah li-ion pack and all the wiring takes up alot of space, and I didn't even add an inverter since it would make it even larger. If you use lifepo4 you will double the size of the pack for the same capacity as a small li-ion pack, lifepo4 cells are giant heavy cells (about the size of a D cell but heavier). Even though these packs operate in 11 volt range most of the time, thats still quite usable, I use my 27ah to run my roadpro cooker (11 amps) they will easily run a 12 volt fridge. On the video they use the flexopower to run a electric drill, so the inverter is quite impressive.
It says it charges at 6 amps, take 6 hours to fully charge. But one thing I don't like is that it only has 5 led's to tell you the battery status. For lithium a percentage gauge would be better. Also from my own 27ah li-ion battery I found out , to fully charge you need to charge at 2 amps or less, When I fast charge at 8 amps, it always stops charging at 12 volts. Thats why a percentage or even a voltmeter is important, so you know if you really got a full charge. They last longer if you don't fully charge but I prefer to get every AH out of my lithium packs. The bms can be overly sensitive in lithium packs and when fast charging will shutoff whenever it sees 12.6 volts or a cell that is too high.
Lithium is expensive but sometimes you do need a small batterypack you can take with you, I have a 220 ah lifepo4 house battery, and 2 smaller li-ion packs 65ah and the 27ah. The 65ah was too big, thats why I built the smaller 27ah.