The principle is the same as von Neumann's "universal constructor", so it doesn't really prove anything we didn't know in theory.
However, von Neumann's design uses a cellular-automaton with many more rules, and those were specifically chosen to help define that constructor (Langton Loops are a more extreme example of choosing rules to make construction easier). In constrast, the rules for Game of Life (GoL) were chosen to be simple and interesting, not fine-tuned for any particular patterns (for an even simpler set of rules, see the Rule 110 cellular-automaton).
We know the GoL is Turing-complete, so it can emulate any computable system; including those other cellular-automata, e.g. von Neumann's universal constructor. Such emulations will typically use a large GoL pattern to represent each emulated cell (e.g. see "life in life"): if we emulate a universal constructor, we can use it to assemble any pattern of those emulated cells. We could also emulate GoL inside some other cellular-automaton, and hence use a universal constructor to assemble any pattern of emulated GoL cells. But the question still remains: can we assemble any pattern of "native" GoL cells? That's what the constructors in the article are doing (at least, for a broad class of patterns).
The rest is a matter of "code golf", trying to make the patterns smaller and faster (and indeed feasible to run on a real PC!)
However, von Neumann's design uses a cellular-automaton with many more rules, and those were specifically chosen to help define that constructor (Langton Loops are a more extreme example of choosing rules to make construction easier). In constrast, the rules for Game of Life (GoL) were chosen to be simple and interesting, not fine-tuned for any particular patterns (for an even simpler set of rules, see the Rule 110 cellular-automaton).
We know the GoL is Turing-complete, so it can emulate any computable system; including those other cellular-automata, e.g. von Neumann's universal constructor. Such emulations will typically use a large GoL pattern to represent each emulated cell (e.g. see "life in life"): if we emulate a universal constructor, we can use it to assemble any pattern of those emulated cells. We could also emulate GoL inside some other cellular-automaton, and hence use a universal constructor to assemble any pattern of emulated GoL cells. But the question still remains: can we assemble any pattern of "native" GoL cells? That's what the constructors in the article are doing (at least, for a broad class of patterns).
The rest is a matter of "code golf", trying to make the patterns smaller and faster (and indeed feasible to run on a real PC!)
https://en.wikipedia.org/wiki/Von_Neumann_universal_construc...
https://en.wikipedia.org/wiki/Langton%27s_loops
https://en.wikipedia.org/wiki/Rule_110
https://conwaylife.com/wiki/Turing_machine
https://conwaylife.com/wiki/Unit_cell