I don’t know where they are getting that. Sounds like the writers should probably have checked with actual science type people.
Lyophilized virus will last for decades with no significant decay in titers. Even in the freezer, these samples should be fine for decades. I’ve thawed similar viral stocks that were at least 20 years old and never even considered that there wouldn’t be exactly the titer listed on the label. Hell, I wouldn’t bet against a sample that fell behind a freezer and was at room temp for 30 years having a bit of viable virus still in it!
Relevant quote from conclusion (admittedly this is for influenza, but I’ve worked with pox viruses and it’s likely similar):
“An archived preparation of virus is usually thought of as a frozen vial of virus suspension stored at -70°C or lower. Our study shows that for the viruses examined here, archival virus stocks do maintain similar viability as when initially prepared. This study further shows that there are other options available if long-term archival storage is not the primary goal and demonstrates the ability to store virus samples
at temperatures greater than -70°C and still have viable virus.”
This study is done by ATCC, and they are the go to people for these sort of things, so a very reputable group.