No

A cyber attack is a cyber operation, whether offensive or defensive, that is reasonably expected to cause injury or death to persons or damage or destruction to objects.

Michael Schmitt et al; Tallinn Manual 1.0, pp 92
  • Cyber Operation?
  • Expected to cause death or injury to people or objects?
Russian Malign Cyber Activity In Estonia Disrupting Government Services does not match this definition. Here's a list of actual cyberattacks and and why this matters.

What happened?

Estonian government networks were harassed by a denial of service attack by unknown foreign intruders, most likely at the behest of the Russian government. Some government online services were temporarily disrupted and online banking was halted. These were more like cyber riots than crippling attacks, and the Estonians responded very well; however, they created a wave of fear in cyber dependent countries like the U.S.