Why did Harry think that the Dursleys would be protected against Dementors until he turned 17?

Question

In Deathly Hollows Chapter 3, Harry tells the Dursleys that they would be in danger once he turns 17 and the protection from his mother’s sacrifice would expire (emphasis mine):

"I hope so," said Harry, "because once I’m seventeen, all of them – Death Eaters, Dementors, maybe even Inferi, which means dead bodies enchanted by a Dark wizard – will be able to find you and will certainly attack you. And if you remember the last time you tried to outrun wizards, I think you’ll agree you need help."

Yet Dudley himself was threatened by a Dementor in Order of the Phoenix! So clearly, they aren't protected from Dementors. Why would Harry suddenly think that they would be protected from them until he turned 17?

Answer 1

The charm only worked on the Dark Lord and maybe his operatives.

The protective charm on the Dursleys’ house kept Harry safe from the Dark Lord and, possibly but not necessarily, those working on his behalf as well. It didn’t protect him from all harm, or even all magical harm.

“While you can still call home the place where your mother’s blood dwells, there you cannot be touched or harmed by Voldemort. He shed her blood, but it lives on in you and her sister. Her blood became your refuge. You need return there only once a year, but as long as you can still call it home, whilst you are there he cannot hurt you.”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 37 (The Lost Prophecy)}}

However, Umbridge was the one who sent the Dementors. Although she was indeed trying to harm Harry, she certainly wasn't the Dark Lord himself ,and wasn’t connected to the Dark Lord at all. She might have shared views with him on wizards born to Muggles, but she wasn’t actually working for him. She sent them independently.

“He never knew I ordered Dementors to go after Potter last summer, but he was delighted to be given the chance to expel him, all the same.’

‘It was you?’ gasped Harry. ‘You sent the Dementors after me?’

‘Somebody had to act,’ breathed Umbridge, as her wand came to rest pointing directly at Harry’s forehead. ‘They were all bleating about silencing you somehow – discrediting you – but I was the one who actually did something about it … only you wriggled out of that one, didn’t you, Potter? Not today though, not now –”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 32 (Out of the Fire)}}

Therefore, even if it would have worked on Dementors sent by the Dark Lord, it wouldn’t have worked in the case of Umbridge sending them.

Also, the Dementors were on Magnolia Road, not Privet Drive - which may be out of range of the charm even for the Dark Lord. While this would be a walkable distance from the Dursleys’ house, since Harry walked there, it wasn’t right near the house. It’s unclear what the range of the Charm is, and at what point the Dark Lord himself would be able to harm Harry.

Answer 2

Harry was just trying to convince the Dursleys to accept protection. However, the Dursleys could have always been attacked.

The Dursleys could have certainly been attacked by Dementors (as evidenced by book 5). Harry was just trying to convince the Dursleys to accept protection now.

The correct question here is why weren't the Dursleys attacked earlier?

It seems that no one magical had any reason to go after the Dursleys. The only reason that anyone would, would be to go after Harry. As Voldemort wanted to kill Harry personally and had no way of doing so in the Dursleys house until his mothers protection wore out, Voldemort saw no reason to go after them. After all Voldemort knew where Harry was. And as a bargaining piece they're quite poor targets as Harry has no love for them.

However, once the magical protection was lifted and Voldemort can kill Harry personally, the Dursleys become prime targets to find out Harry's location. Thus, there is a need to place them under protection.

One can go one step farther and say that Voldemort deliberately left the Dursleys untouched because if he kidnapped them before Lily's protection lifted, they would have no way of revealing Harry's location to Voldemort as they would not be present during Harry's flight to freedom. However, if the Dursleys would be present during Harry's flight, there is a possibility that they would know Harry's location and thus become useful to Voldemort.

Thus the Dursleys only needed protection now although they theoretically could always have been attacked.