As you have mentioned in the question that Pandavas reached to Himalaya through Yoga i.e. by means of Yog-Siddhi. But as we can see from the previous chapter i.e. Chapter No. 1 of Mahaprasthanika Parva, as it is mentioning that all the Pandavas including Yudhisthita walked all the way to their destination. They travelled on foot. But before proceeding they took the vow of keeping the fast all the way as they were on their last journey and on the religion of Renunciation. What Vyasa mean by Yoga here is this vow of fasting and this vow of fasting is called yoga here.
as we can see from the verses below.
अभिजगमुर्बहुन देशान सरिता
सागरांस्तथा ||30 ||
युधिष्ठिरो ययावग्रे भीमस्तु तदनन्तरम |
अर्जुनस्त चान्वेव यमौ चापि यथाक्रमम ||31 ||
Setting themselves on Yoga, those high-souled ones, resolved to
observe the religion of Renunciation, traversed through various
countries and reached diverse rivers and seas. Yudhishthira
proceeded first. Behind him was Bhima; next walked Arjuna; after him
were the twins in the order of their birth; behind them all, O
foremost one of Bharata’s race, proceeded Draupadi, that first of
women, possessed of great beauty, of dark complexion, and endued with
eyes resembling lotus petals. While the Pandavas set out for the
forest, a dog followed them.
This is also more clear from the passage you quoted -
As those mighty ones were proceeding quickly, all rapt in Yoga,
Yajnaseni, falling of from Yoga, dropped down on the Earth.
falling off from Yoga here means because of the vow of fasting.
So all the Pandavas including Draupadi was keeping a vow of fasting till death as they were on their last journey. This vow is called here as Yoga or devoted to Yoga and not as Yoga siddhi or they travelled through yoga. So they all and not only Yudhisthira here are called as yogis. But this did not mean that they were Yoga-Siddhas. The word Yoga is here used in a different limited context to Pandavas vow only and not like we use in general as a practitioner of yoga or becoming a yogi.
Yoga means union and Pandavas were keeping union with fasting that is what the author most probably is intending to say here.