As it is mentioned in the quoted passage, the October 2019 withdrawal was prompted by a Turkish offensive during the Operation Peace Spring. As that offensive ended, the need for evacuation disappeared, and in November of the same year the commander of the U.S. Central Command (in other words, the person responsible for military control of U.S. troops in Middle East), was quoted saying U.S. presence in Syria "not having an end date":
Less than two months after U.S. President Donald Trump demanded all
U.S. troops withdraw from northeastern Syria for the second time, the
general in charge of all U.S. forces in the Middle East now says he
has no orders to leave the region.
“I don’t have an end date,” Gen. Kenneth McKenzie, the commander of
U.S. Central Command, told a small group of reporters in Bahrain on
Saturday.
Roughly 500 U.S. forces will remain in northeastern Syria with their
Kurdish-led partners, the Syrian Democratic Forces (SDF), to continue
fighting the remnants of the Islamic State, McKenzie said during a
visit to Bahrain for the Manama Dialogue security summit. Under
Trump’s directive, the troops will primarily be stationed in the Deir
Ezzor province to guard the region’s rich oil fields, but the Defense
Department insists that the mission is part of the broader campaign to
defeat the terrorist group.
The presence of U.S. troops on the map in question roughly matches the outline of oilfields in North Syria. Official position is that U.S. forces prevent ISIS remnants from gaining control over oilfields; but, from the same article:
Despite the Pentagon’s insistence that the residual force is focused
on defeating the Islamic State, experts have questioned the
credibility of the U.S. mission, noting that the primary threat to the
oil is Russian and Syrian regime forces, rather than the weakened
Islamic State. One key question is whether U.S. troops have the legal
authority to engage Russian, Syrian, or Iranian forces that attempt to
seize the oil fields. The 2001 Authorization for Use of Military Force
does not give U.S. forces the authority to fire on state actors unless
they are acting in self-defense.
And judging by this NYT article - number of U.S. personnel present in Syria increased almost twofold since 2019:
America still has more than 900 troops, and hundreds more contractors,
in Syria, working with Kurdish fighters to make sure there is no
resurgence of the Islamic State, which was ostensibly defeated as a
caliphate in 2019, after five years of wreaking havoc across Iraq and
Syria.
