The vendor is dangerously wrong.
For a given current rating the maximum safe DC voltage will be much lower than the maximum safe AC voltage - usually by a factor of 10:1 or more.
At very low voltages (say 12VDC or less), switches designed for AC mains use will "probably" safely handle DC currents equal to their AC current rating.
Arcs form when current is interrupted OR when voltage is applied to an airgap that is below a given dimension, the dimension being affected by immediately prior history (say less than a few hundred ms) and some other factors.
With AC waveforms the voltage is zero twice in each cycle so any arc that has formed is extinguished at the zero crossing*. For an arc to continue to exist as the switch progressively opens the arc has to reform after each zero crossing, with a progressively larger gap each time. Ionisation of air from a previous arc complicates matters but arc voltage is very roughly proportional to airgap. In an opening switch an AC waveform tends to self extinguish at currents up to and somewhat above the design rating. "Somewhat" is set by the manufacturer and regulatory requirements.
- Inductive loads complicate AC switching arc performance as the current and voltage waveforms are offset, and energy released from an inductive load can generate voltages far in excess of those being switched. Inductive considerations are too complex for easy treatment here, but very inductive loads on AC or DC may require special energy absobtion methods to control arcing during switching.
When DC is switched the voltage is applied continually and, unlike AC, there is no mechanism based on waveform to extinguish the arc on a regular basis. Once an arc has formed it continues as the gap widens and is only extinguished when the air ionisation and air currents formed thermally mean the arc can no longer be supported. For a given fully-open airgap the DC voltage at which an arc will not be sustained is very much lower than for AC.
Arc extinguishing is somewhat of a black art and can be assisted by application of magnetic fields and electrode design such that arcs ascend up a widening gap due to thermal conditions until they self extinguish. Such techniques are not usually employed in switches used in AC mains operated of "extra low voltage" DC equipment.
The switches that you were supplied were almost certainly suitable for 5V switching applications up to the rated AC current, but the supplier was still very remiss in ntheir claims.