The frame of the bike is made of tubes. Fine. These tubes are welded together at the joints. The interior of the TUBES is hermetically sealed.
Stir welding, does not require material (cost). It's stronger and faster. The tube is fixed in a jig joint side up. Then stir welded as a 100 ft section ect. Cut into individual parts. Then assembled via traditional welding processes. There are no breather holes.
If there were, then the inside of the pipe would have to be protected from exposure to O2. Which costs money. Thus, design the process such that you minimize that additional material and work.
My family has been in the aluminim casting business for over 50 years. I've grown up in factories as my family owns several manufacturing firms. One which produces hospital beds and is the largest supplier of cots and emergency medical supplies to the US government (FEMA).
Pipe, tube.... what the hell ever. Apples and oranges. Fill either with water, seal it, and freeze it, and you will not get anything like what you see in the picture.
I consult as an SME to automotive mfgs for process integration and quality control for cast aluminum blocks. Yes, they can be alloys or whatever, but it's like 5% other metals for the given application. Negligable in this case, as I can say without a doubt that bike is NOT AN ALLOY. I say this because they purchase their raw materials from my uncles firm. I do the IT and security for most of my families firms. Their purchasing is automated into our systems and they most certainly do not purchase alloy composites of any type. 99.99% galvanized aluminum.
Finally, I've been in most of these factories in China and the US. I've done destructive testing and worked in classified mfg for years. If you want to get into the science of how this image is most certianlty the result of dynamic loading on a truss that is load bearing let's go.
If fact, I have hundreds of papers on my computer about materials and structural failures, because I get paid to look at a piece of something, like a missile fragment, and tell you exactly the forces and circumstances that brought it to this state. How it was mfg and beyond that, how tight their production controls are ect. I can tell you if two given materials were produced in the same region simply by the affect the atmospheric pressure and earths magnetic field imprint in the lattice as it crystallizes.