Just as with the gold on the temple, these hypocrites put more emphasis on the material gift than the spiritual altar that sanctified the gift. Jesus revealed that the motives behind the gift are more important than the gift itself (Luke 21:1-4 and 1 Corinthians 13:3). An inferior material gift given with the proper motives is recognized by God as superior to a larger material gift given with the wrong motives.
Rather, this verse is all about ‘oaths’ - in particular the basis of, or what was used to ‘bind’ the oath. In this case, the difference between using the ‘temple’ - a spiritual value that they didn’t understand, therefore because they had no understanding of this, i.e. therefore this would have meant nothing, so this made it worthless as a ‘anchor’ for the oath.
Where as they did understand gold - a material value - therefore this made or would have given the ‘oath’ a meaningful foundation.
In the Hebrew way of thinking, an oath was a solemn appeal to the truth of an affirmation or promise. The Law of Moses established guidance concerning oaths in several situations: the loss of one’s personal property (Exodus 22:10-11), the finding of lost possessions (Leviticus 6:3), when a woman was suspected of unfaithfulness to her husband (Numbers 5:11-31), and the misuse of swearing and concealing the truth (Leviticus 5:1). All of these were ‘real’ (non spiritual)
The Old Testament also cautioned against swearing lightly with the solemn warning of Ecclesiastes 5:5, “Better is it that thou shouldest not vow, than that thou shouldest vow and not pay.” Gold therefore having ‘real’ value, it’s ‘loss’ would have an impact - makes the ‘oath’ valid.
Where as the temple would have had no such ‘bind’ on the oath.