I'm not sure who the 'they' are that you reference to, but China gave the WHO information, which the WHO passed on, exactly as they are charged to do.
If you have a problem with that you should probably lay it at the feet of the Chinese government officials that did that. But Adhanom did visit and did in fact get the Chinese to cooperate.
The Taiwan/China issue is obviously a very sensitive one to the Chinese and this has muddied the waters, as has - probably - an initial attempt by China to wipe this under the carpet. The end result is that the pandemic has probably become worse than it could have been but:
- Subsequent events, even after much more knowledge had been gathered by the West did not lead to substantially different policies in Western countries, if anything the countermeasures were - and still are - too late and too lax.
- The WHO does appear to have learned some lessons from this pandemic.
- The prevention paradox is real: if the WHO had sounded a massive alarm early on and had been found to be wrong then they would have been hard pressed to do this again at some point in the future because everybody would have called them the boy that cried wolf. But this time there was a wolf, and this time wasn't helped by the previous times that WHO raised the alarm and no pandemic at a massive scale occurred. Note that it doesn't really matter whether we prevented it successfully or whether it would have fizzled out.
- Finally, it is super easy to criticize from the sidelines, I think the WHO has a nearly impossible mission, to get a very large number of countries to collaborate in spite of a whole raft of political complications, egos and other bullshit. Given that they did a commendable job, I hope they will put the lessons learned to good effect, they did more than any single government in the world to help combat this pandemic.