Understandably, there has been widespread disbelief on social media regarding the claim published by the Islamic State's Amaq News that the Las Vegas attacker was a "soldier of the Islamic State" (i.e. inspired by Islamic State to carry out the attack). Probably anticipating or responding to the disbelief, Amaq News released a follow-up post saying the attacker had embraced Islam months ago. Amaq News only purported to rely on "a security source." In this context, "security" refers to the carrying out of operations inside enemy territory. Thus, any Islamic State-directed or Islamic State-inspired attack inside the U.S., which is leading the coalition against Islamic State, would come under this category.
Amaq News, established in 2014, is almost universally recognized as a part of the Islamic State's media apparatus, but it has not been formally acknowledged as such by the Islamic State in its propaganda directed to the external world. Thus, Amaq News represents the foremost example of an auxiliary agency for the Islamic State. In not being formally acknowledged to the external world as a part of the Islamic State, Amaq News can present preliminary claims of attacks as news agency items, thus the framing of a "source" speaking to Amaq News on these matters.
Most of the Islamic State attacks in the West and other enemy states are first reported on via Amaq News. Many of the claims turn out to be reliable with further evidence provided: for example, Amaq News has released exclusive videos of attackers. These cases suggest the attackers have been in contact with Amaq News, perhaps via personnel or intermediaries operating in the West. The Amaq News items can also be followed up by official Islamic State media statements.
It is certainly true that the Islamic State does not claim every attack that results in large loss of life or might even be plausibly attributed to the Islamic State on intuition. The St. Petersburg metro bombing in April 2017 is a clear case-in-point. However, since Amaq News is not formally acknowledged to be a part of the Islamic State, the outlet may also be used to make a tentative claim to an attack. If no convincing evidence ever emerges to show an Islamic State hand or inspiration after that, the Amaq News statement can be left as it is with no follow-up in formally acknowledged Islamic State media organs. This saves the Islamic State the trouble of having to 'retract' a claim, since from the standpoint of communicating to the external world, no formal claim was made if the only material ever issued was an Amaq News report relying on a mere 'source'. Some examples of this behaviour include some purported Islamic State assassinations carried out inside Turkey, and a casino shooting in June 2017 in the Philippines. Even so, the Amaq News item itself can be enough to make a psychological impact and stir up emotions, with many eager to seize on whatever they can find to support their own preconceptions regarding an attack.
The best advice for any claim emerging via Amaq News is always to wait and see for subsequent evidence. A level-headed approach is the order of the day, not irrational fearmongering that plays into the Islamic State's hands.
(Update): The attack has now been formally claimed by the Islamic State:
"By God's grace alone, responding to the call of the Amir al-Mu'mineen Sheikh Abu Bakr al-Husseini al-Qurashi al-Baghdadi- may God protect him- to target the states of the Crusader alliance, and after exact observation of gatherings of the Crusaders in the U.S. city of Las Vegas, one of the soldiers of the Caliphate (Abu Abd al-Birr al-Amriki: may God accept him) lay hidden armed with machine gun weapons and various ammunition in a hotel overlooking a concert, so he opened fire on their gathering, leaving among them 600 killed and wounded, until his ammunition was used up and he departed as a martyr as we reckon him and God is his reckoner. And glory belongs to God, His Messenger and the believers but most of the people don't know, and praise be to God the Lord of the Worlds."