The correct response, as it stands presently is NOT this.
So this interpretation is misleading and is wrong. To be clear - Chelsea cannot buy or loan, or extend the loan of anyone whilst the ban is in effect (and this includes buying, or extending the loan of Higuian).
What the Guardian article is actually exploring is something entirely different, it is what Chelsea might do this summer on the assumption that they appeal (which they have done) and that the appeal takes so much time to process that the final outcome is still pending in summer 2019.
Note the actual words here in your quote, "for fear of the ban eventually being upheld." You should read the entire article which prefaces your quote with "the case will go to the Fifa appeals committee and, if necessary, the court of arbitration for sport and could drag on for a considerable time, with the enforcement of Fifa’s ban almost certainly stayed pending the appeals process. That buys the club time."
TLDR: if the ban is upheld prior to summer 2019, Higuain is 100% leaving Chelsea. So, the only way Chelsea buy Higuain is if the ban is postponed pending a final decision from CAS and this has nothing to do with him being a 'new' signing or any other such imaginary loophole.