The closest thing to concrete data about that whole situation could accurately describe:
-Agency plugs in lenovo laptop with preload intact
-Agency notes that a TCP SYN packet was sent to China, but not allowed to actually get there.
-Agency says 'screw it' and bans it without further analysis
This could be nefarious or it could be checking for firmware or driver updates. There's no way to guess what really happened without details of any investigation coming to light.
Keep in mind that it was likely an activity driven by some agenda. Notably, these agencies start from a perspective of 'distrust china' and consider it their job to prevent that vendor selling into agencies. So they seek the flimsiest reason to hold up to impose a ban, which no one really objects too hard to since it's politically better to not source from China anyway. The agencies may not have detected a real threat, but they likely presume a real threat is a significant possibility that they have no way of practically detecting, so they run with this.
If there was an unambiguous backdoor seen, you bet your ass the agencies would be shouting from the rooftops. Instead, they are doing enough to keep it away from sensitive areas, but not so much to invite much scrutiny.
Finally, if China *really* wants backdoors, they don't need to actually have even slight ownership of the company. All the big companies gleefully hand over pretty much full control of their manufacturing and much of their hardware design, software, and firmware development to China anyway. The nationality of the CEO means approximately nothing in the scheme of state sponsored espionage.