The overlap comes in the form of militarization of the police (Republicans give police ridiculous equipment or powers that the Democrats refuse to limit or rescind because they don't want to appear "soft on crime"), the proliferation of punishment instead of prevention (Republicans push policy to criminalize various things which the Democrats then quietly use for the same reason as above), the expansion of the surveillance state (just look at these stupid device age verification laws) privatization of things that need to be public services because they have to run at a loss (many senior members of the Democrats are at odds with younger members over their support for the Republican initiated ideas of partial or full privatization of Amtrak and information services like NOAA), trying to wield the judicial branch against the executive (Republicans raise things to the Supreme Court to make precedent while Democrats try and keep things in the circuit courts and prevent appeals) and others. While their ideas of social reform, bodily autonomy, and worker protections have progressed, in terms of finances, judicial enforcement, and foreign policy the Democrats haven't meaningfully changed their stances since 2008. Much of this is the same as back then when both parties showed support for the ridiculous punishments for digital piracy, the PATRIOT Act, and the plans to replace the Post Office with something like DHL.
The main idea of this stagnation leading to complicity comes from the fact that most often when the Republicans break something or introduce new ways to harm a given group the Democrats never really do anything visibly to reverse that. That idea had been bubbling under the surface since Chuck Schumer became Senate Majority Leader in 2015, but gained a lot of traction after Biden's term because although most of the first Trump era's policies had been implemented by executive order or by appointed department heads most of those executive orders were not nullified. Of the 33% nullified about half of those were inconsequential such as EO14016 revoking EO13801 which was about apprenticeships, and those appointed department heads such as Mike DeJoy were not relieved of their posts despite demonstrable harm. There's a reason we have the joke that the only thing Biden did was make Juneteenth an official holiday and gave people two grand to buy gaming PCs.
There are a lot of things that Democrats have actually done beneath the surface such as strengthening marriage protections for LGBTQ+ people, the carbon tax credit, bodily autonomy, giving aid and leniency to migrants and refugees, and pushing for student loan forgiveness. But all of that was so visibly stripped away in a spectacle, and that makes it hard for people to believe the Democrats are actually doing anything because if they were wouldn't all of those be more robust and defensible? The primary example people point to is Roe V. Wade, where the Democrats had forty nine years to create several layers of legislation to protect what Roe V. Wade had initiated, and yet did nothing. With all of this it creates the perception that the Democrats never really do anything and are thus stagnant.