Ah, they make a good point. If you spend a 'quantum resistant' UTXO from an older address type the pubkey is visible to the world while that transaction is waiting to get mined. So you better set a high enough fee rate to get it mined before some quantum computer cracks it.

In practice, this time window will be short under normal circumstances and most users will be spending small amounts, probably not worth the cost to crack the privkey.

https://github.com/bitcoin/bips/blob/master/bip-0341.mediawiki#cite_note-2