Well spotted! Although T'Pol's words can have several interpretations.
T'POL: We held back their warp programme for one hundred years. A policy which you supported. If we would have helped them develop faster ships, better defences.
My interpretation is the Vulcan's didn't hold back the Warp 5 engine by sabotaging research or misleading humans, instead they didn't share technology or knowhow which could have accelerated the Warp 5 project.
So humans didn't artificially develop Warp 5 capability at a fast rate than they should have they instead developed it at their own pace. It depends on your viewpoint whether you interpret this as the Vulcan's 'holding back' but personally I don't.
To be fair, when T'Pol said that, she wasn't exactly her fully logical self and was more emotional than ever.
On the other hand, it seems to me that if this statement from T'Pol if accurate (and seems like it is), and also factoring in what Archer mentioned that Vulcans refused to share their findings, it was a COMBINATION of factors.
Namely, Vulcan's weren't too thrilled about Humanity joining the interstellar stage as they didn't deem them 'ready' for it, despite seeing Humanity solving their own problems in only 50 years since First Contact... in fact, this right there may have worried them all the more, because they weren't accustomed to seeing this speed in social change (wait until they learn we can undo damage to Earth in reality from climate change and restore full biodiversity, clean up the planet and create a create a FAR higher living standard for each and every human alive with 10-100x lower footprint on Earth in less than 10 years with EXISTING technology in early 21st century already - I think some might even have a small Vulcan stroke).
So, the way I see it, Vulcans had influence over Earth space program and Starfleet. They would make recommendations and the higher ups on Earth would listen because Vulcans are experienced interstellar species.
Holding back the space program would have meant something akin to hanging onto every possible error that Vulancs uncover in humans experimenting with new technology and having us running 'endless tests' over and over (much like it was suggested when we saw flashbacks from breaking Warp 2 barrier).
Add in Archer's comment about Vulcans not sharing their own research with Earth indicated that Vulcan sensors and scientists may have picked up something Earth sensors and scientists did not, and it would have helped Humans minimize other mistakes.
Vulcans didn't like us taking risks with technological advancement (part of which attributed to its acceleration) and thought we were doing things too fast.
Now IF Vulcan's hadn't interfered as much as they have in Earth policy but also didn't share their findings, its possible that Earth would have taken more risks with Warp drive and may have ended up developing Warp 5 earlier than they have... or it would have happened at the same rate.
No 'endless tests' from Vulcans would mean Earth would have tried to break the Warp 2 barrier earlier, which meant what we saw happen with the Warp 2 testing would have happened to other candidates... and Warp 5 ships would have taken off maybe a decade or two earlier.