Tesla bought BTC when it was around $36k IIRC, so even with the value decrease yesterday-today, they're still up significantly.
Also, liquidity today doesn't say anything about liquidity in a year's time.
They stated clearly that putting less then 10% of their cash assets into something that was highly liquid, but had also chance at gaining value was preferable to cash.
Specially because they can make it liquid difference currencies very fast.
[1] https://finance.yahoo.com/news/musk-says-tesla-sold-bitcoin-...