All I can say is that Wikipedia is not a reliable source but I understand investigating profoundly every minor topic is a nuisance, but if you don’t, it all basically boils down to “trust me bro” which is what my answer is about to be:
Jose Luis Espert is a Libertarian Legislator sure, but not on Milei’s Party, he and her companion are their own block “Avanza Libertad” while Milei’s block is “La Libertad Avanza” which Understandably might cause confusion, but the fact is that they arent the same. Meanwhile Espert’s block is affiliated with “The Pro” or “Juntos por el Cambio” who Milei’s government has made a coallition with to be enabled to govern/pass laws in the Senate. Basically an alliance
Espert is NOWHERE in that picture however. The english article might just be outdated, tho as Milei and Espert used to be friends but they went separate ways when Espert Joined the Pro like 7 months ago. Would recommend the spanish article which is somewhat more accurate