Summary
- Yoda considers Luke to be the last Jedi because he adheres strictly to the Jedi Code and does not accept unconventional training methods like those used by the "Bokken Jedi."
- Yoda believes that the "Bokken Jedi" have deviated from the path of the light and are heading closer to the dark side due to their aggression.
- Yoda does not know if the "Bokken Jedi" have faced their trials and become true Jedi Knights, so he cannot consider them to be full Jedi like Luke.
Master Yoda tells Luke Skywalker that he's the last Jedi, despite Order 66 survivors still lingering in the galaxy – and Star Wars has explained why that is. As one of the Jedi Order's oldest and wisest Jedi Masters, Yoda has come to understand the Jedi Code just as well as he understands himself. This has caused Yoda to set very high standards for all ranks of Jedi in a way that's managed to misguide him in the past, most notably during the Clone Wars. Nevertheless, he still serves as a proper and wise master for Luke to finish the training that Obi-Wan started, something that sees Yoda consider Luke to be the last remaining Jedi.
This statement has made viewers scratch their heads ever since Order 66 survivors such as Ahsoka Tano herself were revealed to still be living during that very same time period. While Yoda may have been in exile on Dagobah, he still would have been fully aware of the Order 66 survivors who continued to make their way through the galaxy, as well as the other Force-sensitive individuals who sought training from these survivors. The question of why Yoda would make such a claim about Luke with all these other Jedi still running around the galaxy has been raised for years, and now, Star Wars has explained why that is.
Your browser does not support the video tag. Baylan's "Bokken Jedi" Insult Fits With Yoda's Views 
Baylan Skoll is the one who assigns Ezra Bridger the insult of "Bokken Jedi," named after the wooden sword with which the Jedi train. In associating the term with someone like Ezra, who was born after the Jedi Order fell and trained by Jedi Knight Kanan Jarrus, Baylan diminishes him to the most basic level of what a Jedi could be. Just as the bokken is a mock lightsaber, so is Ezra a mock Jedi, one who seemingly takes the title without actually having any of the same power or purpose.
This perspective actually fits with the same views Yoda holds, as he would have also adhered strictly to the Jedi Code in terms of who would be considered a true Jedi. Yoda would not have approved of the unconventional ways in which these "Bokken Jedi" have been trained, even if it's by those who experienced quite a good chunk of their life at the Jedi Temple. The Stupendous Wave explains this in more depth, focusing on why Yoda didn't consider Jedi like Ezra, Ahsoka, and Kanan to be "true" Jedi.
