Probably banking on Trump being impeached before the phone launches.
Declaring that they would discontinue supplying Huawei/Hisilicon was likely a precautionary measure while they figured out the exact legal position they were in.
ARM's is a much murkier position vs AMD and Intel considering the lions share of their IP is owned by a non-US company to begin with.
It may be that they simply told Huawei what was US licensed tech alone and told them to fill in the gaps, or ARM developed equivalent tech to fill in the gaps themselves.
On the other hand, ARM may have simply pointed out to the US government that one of their foremost semicon companies Qualcomm would be crippled in the high end market if ARM decided to go with the nuclear option to protect their freedom to license - ie deny Qualcomm licensing for new CPU cores.
Or ARM simply got a waiver because whatever US licensed tech they use was so inconsequential and common place that the the govmt would look like fools for blocking them on that alone.