Interesting, I found that page. Why not give it a try? It’s a very, nay extremely, competitive business in China with multiple PCB fabs battling it out. I usually find that the shipping is the greatest part of the landed cost, I end up paying about $1.5 - $2 per board in quantity 10. I’m willing to wait 3-4 weeks instead of paying expedited shipping.
Well a prototyping board is used to experiment and verify the circuit idea. You should be fairly confident the design works when you commit to PCB, because of the long turnaround. Then too you spend time soldering and you don’t really want to discover a design error after you have assembled the board.
Edit: I registered an account and found that the free offer is for 5 boards, < 100x100 mm, 2 layer, 1.6 mm FR4, other settings normal, including green soldermask and white silkscreen and surprisingly, or maybe not, HASL lead free. Any deviation from these values and it’s not free any more. Also the lead time is 20 days when the norm is 2-3 days. So my surmise is that they are offering this by using the unused areas on their large panels of paid orders, hence the 20 days wait until some free area becomes available. Maybe the 20 days is an overestimate, but beggars can’t be choosers.
If you can accept the wait, then great. Otherwise there are many other houses promoting $2 for 5 boards, shipping on top, and you can have other soldermask colours.