To be on a shoestring means to be on a very tight or limited financial budget, but in this context they were living very lavishly
Context: "They had suddenly gone from comfortable to living very lavishly and within a large financial budget and being behind on something as important as their mortgage." Claim: "They had suddenly gone from comfortable to living on a shoestring and being behind on something as important as their mortgage."
Considering the context, is the claim true, false, or unknown?