It says “after they had understood it” and doesn’t imply writing.

This is backed by 2:74

Even then your hearts became hardened like a rock or even harder, for some rocks gush rivers; others split, spilling water; while others are humbled in awe of Allah. And Allah is never unaware of what you do.

It seems to be referring to present tense. It goes on:

2:75

Do you ˹believers still˺ expect them to be true to you, though a group of them would hear the word of Allah then knowingly corrupt it after understanding it?

So, a group of them corrupted it. Only a group would corrupt their beliefs.

2:89 goes on to make it clear they still had the scripture “in their hands”

Although they used to pray for victory ˹by means of the Prophet˺ over the polytheists, when there came to them a Book from Allah which they recognized, confirming the Scripture they had ˹in their hands˺, they rejected it. So may Allah’s condemnation be upon the disbelievers.

In addition, the Qur’an does ask it’s followers to check with Christians on if it’s true or not.

Surah 10:94

If you are in doubt about what we have revealed to you, then ask those who have been reading the Book before you.

I also apologise for my typo. I meant 5:47, not 4:57 and have amended it