Two claims about some purported ISIS-régime alliance refuse to die. One can be disposed of in short order. Yes, too-clever-by-half Syrian intelligence agencies supported various extremist Islamists in 2008-2011 or so. But this is hardly the same as supporting ISIS, which had no independent identity back then. It's not even as damning as, say, Israel's early support of Hamas. No one supposes Israel and Hamas are, therefore, buddies in secret.
The other claim requires only slightly more attention. With the dogged sophistry of 9-11 conspiracy theorists, some still hold that, if now IS and the Syrian régime do fight one another, this is new. Before, they were in a tacit alliance. They didn't really fight one another. Indeed the régime and IS ganged up on the rebels.
It's true that the Syrian régime, not being insane, probably did on occasion see in IS attacks on the rebels an opportunity to mount their own assaults. But to take advantage of fighting between your enemies doesn't mean they're not your enemies. The reason for the current régime campaign against ISIS has nothing to do with a change in alignment. It has to do with obvious strategic priorities.
The régime, all along, has fought its battles where it was most threatened. This meant securing, as much as possible, its coastal enclave (Latakia/Tartous), Damascus, and population centers near Damascus. That's why it made little effort in the extreme South, where the rebels did well, or the remote Northeast, where IS established itself in Raqqa. In the Northwest it lacked the resources to retake Aleppo at a time when that would have meant confronting the US-backed rebels, the Kurds and perhaps Turkey.
With Russian support, things have changed radically. Homs, once the most important rebel stronghold in the core region, is under régime control. The Western enclave is secure. Even the less important areas are no concern. In the South the rebels' US and Jordanian backers are interested only in diverting anti-Assad militias into anti-IS proxies. US support in the Northwest has come to have much the same objective. What's more, America's (and Russia's) Kurdish allies can be counted on to neutralize the rebels not only in Aleppo, but all along the Turkish border. Russian support also means far fewer casualties for the Lebanese and Iranian-backed Iraqi militias, so that Assad no longer has manpower problems. The one remaining problem area is Idlib, where Jabhat al Nusra refuses any truce with the régime - but there the US-backed rebels know they cannot support a Nusra-initiated offensive without losing their backing. This means that the danger from Idlib is very moderate. Finally Russia is very successfully making problems for Turkey via the Kurds, so that Turkey, and therefore Gulf State powers who supply the rebels through Turkey, are in no position to give the rebels serious support.
That's why the régime fights IS now, and why it didn't fight IS as much before. It has nothing to do with alliances or cooperation. It's because the major threat to the régime, the rebels, became a very minor threat. The change in strategy occurred as soon as that became clear.
The insistence in the face of these indisputable facts that IS and the régime have some covert relationship, or some de facto alliance, is disturbing, because it is so clearly false. When the rebels' supporters display delusional behavior, it can hardly help what's left of the rebels' cause.