They probably will use an Nvidia chip, every leak suggests they're going to use the T239 and even if they don't use that chip specifically it would make sense to use an Nvidia chip, because like you said, nobody else really makes good mobile gpus.

Way back in June 2021, noted technology leaker kopite7kimi posted a detailed picture of Nvidia's T234 processor, revealing for the first time that Nintendo would be receiving a customised variant, dubbed T239. In the two years that followed, a wealth of overwhelming evidence has essentially confirmed that they were right. The T239 is an advanced mobile processor, based on an octo-core ARM A78C CPU cluster, paired with a custom graphics unit based on Nvidia's RTX 30-series Ampere architecture, combined with some backported elements from the latest Ada Lovelace GPUs - and with an all-new file decompression engine for fast engine. It also supports Nvidia's console-specific graphics API, all but confirming that it's destined for the next generation Switch.

https://www.eurogamer.net/digitalfoundry-2023-inside-nvidias-latest-hardware-for-nintendo-what-is-the-t239-processor