As was mentioned in other answers, the information on the screenshot from the first source is just plain wrong (all the verbs marked red are in fact perfective).
However, I'll expand a little bit on the claims in your post:
- … the prefix по- with a multidirectional verb, still gives an imperfective,
- … it makes the verb perfective.
Normally, mechanically adding just about any prefix to an unprefixed imperfective verb does make it perfective — usually with a change of meaning. Думать, an imperfective verb, means "to think"; задумать, придумать, передумать, выдумать, all perfective, mean, respectively, "to conceive (an idea); to come up with; to change (one's) mind; to make (something) up". All related to thinking, of course, but only tangentially. It's not "the same word with just the aspect changed".
Among the exceptions to this rule, there are two major classes of verbs:
Verbs formed with the suffixes -ива/-ыва: задумать/задумывать, оплатить/оплачивать etc. A verb thus formed will usually retain the core semantics of the original, but will acquire a new aspect, so, for these words, it usually is "just the aspect change". Traditionally these verbs are called "iterative", but that's just one, well, aspect of them.
There are several (14 or more, depending on who's counting) Russian imperfective verbs of motion which don't accept these suffixes. They form their "iteratives" through other means: other suffixes, root changes or even suppletion: идти/ходить, нести/носить, лететь/летать etc. The members of these pairs are usually called "non-iterative and iterative verbs of motion" (моторно-некратные и моторно-кратные глаголы), but are indeed sometimes also referred to as "unidirectional and multidirectional".
So what sometimes happens when you slap a prefix onto this "multidirectional" version, is that the result remains imperfective, but acquires a completely new meaning — sometimes only vaguely related to the original one: перегонять "to pass (in a race)", переводить "to translate", заводить "to start (an engine)" etc.
However, the very same multidirectional verbs can also form perfectives by adding prefixes, same as any other verb, resulting in forms like поплавать "to have a swim", вы́ходить "to tend, to nurse (someone back to health)" etc. These verbs are also only remotely semantically related to their root forms.
Sometimes these two venues of word formations form homographs or even homonyms:
- свозить "to bring to one place (impf.) / take someone for a tour or a ride (perf.)"
- налетать "to attack suddenly (impf.) / to have flown enough (for something) (perf.)"
- выноси́ть "to bring out (impf.)" / вы́носить "to carry a child (perf.)"
, and so on.
In this sense, both the claims you mention are accurate.