That it is false can be easily proven by listing a few of the bazillion things which are not made explicit in Scripture:
It is un-Biblical to assume that Scripture alone can resolve religious questions, because the Bible itself refers to sources of Apostolic truth beyond what is in the written text. For instance, the Apostles' example:
Philippians 3:17
Join with others in being imitators of me, brothers, and observe those who thus conduct themselves according to the model you have in us.
Or here,
2 Thessalonians 2:15Or here, where Jesus says the Church itself has the authority to make rulings:
Therefore, brothers, stand firm and hold fast to the traditions that you were taught, either by an oral statement or by a letter of ours.2 Thessalonians 3:6
We instruct you, brothers, in the name of Our Lord Jesus Christ, to shun any brother who conducts himself in a disorderly way and not according to the tradition they received from us.
Matthew 18:17
If he refuses to listen even to the Church, then treat him as you would a Gentile or a tax collector.1 Timothy 3:15
The church of the living God, the pillar and foundation of truth.
OK?
But let's look to God's Word in the Old Testament for many an example of holy veneration of human persons, which is not the same as adoration due to God alone. This is something that wonderfully permeates Old Testament culture.
So, veneration? Totally Biblical.
So, then are you saying that I can venerate Mary without praying to her and/or considering her in some way an integral part of the redemption process, and you will honor that position as one of those that is not settled by Scripture?
8I, John, am the one who heard and saw these things. And when I heard and saw, I fell down to worship at the feet of the angel who showed me these things. 9But he said to me, Do not do that. I am a fellow servant of yours and of your brethren the prophets and of those who heed the words of this book. Worship God. Revelation 22:8-9 NASB
No one has claimed everything we do daily or every action we do daily is in the Bible.
Does the Bible say it's ok to own a car?
See how ridiculous the argument is?
However, in matters on how one can come to faith in Christ and what that means the Bible addresses that.
How we conduct ourselves before God and others is covered.
To your assertion...It is un-Biblical to assume that Scripture alone can resolve religious questions...
14You, however, continue in the things you have learned and become convinced of, knowing from whom you have learned them, 15and that from childhood you have known the sacred writings which are able to give you the wisdom that leads to salvation through faith which is in Christ Jesus. 16All Scripture is inspired by God and profitable for teaching, for reproof, for correction, for training in righteousness; 17so that the man of God may be adequate, equipped for every good work. 2 Tim 3:14-17 NASB