Depends on what you consider a "shooting". Typically, the media is referring to the Bath School Disaster of 1927, in which 44 people died, including 38 schoolchildren. Most of the deaths in that attack were caused by bombs, but several people were shot to death as well. (Source.)

If the meaning of "shooting" is expanded to include domestic rebellions, insurrections, and outright battles, then any of the recent mass shootings is far from the top of that list. A few "not modern" examples:

• The Stono Rebellion occurred on September 9, 1739, in the colony of South Carolina. It was a slave rebellion in which approximately 75 people were killed. (Source.)

• The Nat Turner Rebellion occurred from August 21-23, 1831, in Southampton County, Virginia. There were approximately 175 black slaves and white militiamen killed in the rebellion itself, with an additional 56 slaves executed after the rebellion was put down. (Source.)

• The New York Draft Riots occurred during the Civil War, from July 13-16, 1863, in Manhattan, New York City. About 120 people were killed in the riots. (Source.)

• The Colfax Massacre, in Colfax, Louisiana, on April 13, 1873. A force of 300 armed white men killed somewhere between 62 and 105 black men and white Republicans in a bid to overthrown the parish government. (Source.)

• The Wilmington Insurrection of November 10, 1898, saw a mob of 2000 white men overthrow the democratically elected government of Wilmington, North Carolina, upset that the city council just elected for the next term was to be 1/3 black. As many as 60 black residents of Wilmington were killed in the riot. (Sources 1, 2.)

• The East St. Louis Riots of 1917. A mob of 3000 white men marched downtown attacking minorities, after a union meeting in town about the perceived threat of competition from black out-of-towners moving to the city and competing for jobs. The riot left as many as 100 victims dead, both black and white. (Source.)

• The Battle of Blair Mountain occurred between August 25 and September 2, 1921, in Logan County, West Virginia. It was the biggest labor uprising in U.S. history, and a legitimate battle. 10,000 coal miners organized and confronted a force of 3000 strikebreakers and law enforcement officers and the battle resulted in as many as 110 deaths. (Source.)

So the media's language about "modern" history is, first of all, to distinguish it from the Bath School Disaster, in case the raw numbers being reported do not exceed that disaster. But secondly, it's to distinguish the mass shootings from earlier armed incidents of civil unrest, including labor uprisings, slave uprisings, and white supremacist uprisings, and other uprisings that occurred before World War II.

It would now be appropriate to call this the deadliest even from a single perpetrator ever?

The events listed are about groups of people. It seems much different when its a lone wolf than a mob.

For what it's worth OP, this is a stylistic expression that would probably be better directed at those in the journalism field itself. /u/j1375625 had a great answer from a historian's perspective, but I don't think you can say that's definitely why the media uses 'modern history' and not just 'history.