This claim is patently false. Even though outside spaces are safer than indoors, it's still possible to spread covid-19 outdoors, moreso if you do not respect social distancing.
https://www.bbc.com/news/explainers-55680305
https://www.mayoclinic.org/diseases-conditions/coronavirus/i...
It seems that anti-maskers grab the claim that masks are not necessary if you're sitting quietly at a table on a plaza on a sunny day while preserving the min >2m social distancing, but completely disregard all other scenarios such as the journey to said plaza and unexpectedly finding yourself in high-risk scenarios while you move around.
To put things in perspective, there are documented cases where COVID spread through person's that stopped to chat outdoors while not wearing a mask.
What makes this sort of argument regarding the use of mask particularly dangerous is that it starts with a premise based on a very specific scenario combining key factors that are known to be safe (i.e., sit outside without a mask on a sunny day while preserving social distancing) but ends up trying to justify discarding each and every key safety factor (social distancing while outdoors in sunny, hot days, and even the need to wear a mask while going to a public place).
Lastly, although wearing a mask is important to keep you from catching COVID (or any airborne respiratory disease) the main goal of wearing a mask is to ensure that asymptomatic people (and specially symptomatic people) don't unwittingly spread the virus.