So while Anandtech quotes Apple's figures (which are exactly the same as I estimated), they don't offer any reason to dispute those figures. Obviously testing the chips will determine the veracity of those claims.
As for the M1 GPU real-world gaming benchmarks on games like Borderlands 3, Total War: Three Kingdoms, Tomb Raider and PUBG — these are all coded for Intel chips and being run in Apple's Rosetta 2 translation layer — they are in no way reflective of games coded natively for M1 on Mac as they aren't any AAA titles ported natively for the platform yet that I am aware of (Minecraft notwithstanding).
As for Apple's performance claims, sure, pinch of salt. But rarely, if ever, does Apple overstate its claims. As an example, recently it claimed the A15 is 50 percent faster than the competition (i.e Snapdragon 888). Turns out testing showed it is actually 62 percent faster than the S888. Given the performance of the M1, I would be surprised if Apple's performance claims are overstated.